And that didn't have to be the end of the story either. As I said before, nobody took the idea of the "Fall of Rome" seriously at the time. And the Western Empire still had enough steam as a political entity after that to repel a Hunnic invasion.
Thesis: Had the imperial court treated the Gothic foederati as potential provincials rather than outsiders, the West could have evolved into a federative Romano‑Germanic commonwealth—and Justinian, by restraining his reconquest, might have preserved the unity needed to resist later threats.
Not even 20 years before the official collapse of the Western Empire, Rome was still able to manage such a feat. I think that is a testimony to Rome to rally so many Germanic tribes. After all, many of the Germans (probably not the Vandals...) did not view themselves as replacing Rome. They viewed themselves more as continuing the idea of Rome, quite similar to how the United States is often viewed as an idea. Though we get the occasional viewpoint of that the United States is not an idea (but a nation, speaking in a philosophical sense)...and I believe that is what happened with Olympius; he rejected the idea of Rome or very least refused to expand it to include the Visigoths who were more than willing to become Romans, they just wanted to be treated properly. Let's not forget the Visigoths originally came in as refugees, not conquerors. The Senate wanted to accept Alaric's terms, which shows me that the fear of a Germanic takeover may have been overblown by Olympius (which to your point, he may have genuinely feared, but again, I think he was misguided as the Senate was opposed to his view).
Many of the intellectuals in the Eastern Roman Empire never considered Western Rome falling, still considering Italia a provincia and Odoacer and later Theodoric as "our patricius". They all viewed the various Germanic tribes inhabiting former Western Roman territory as a continuation. After all, Odoacer paid tribute to Constantinople even when they had no reason to, only to be
Roman and the German leaders styled themselves as continuation of Roman rule (as many of the administrative tasks were done by "former" Roman officials.
Interestingly, when Odoacer sent the regalia to Zeno, Zeno accepted it and made him viceroy of Italy, ruling Italy on behalf of Rome. It wouldn't be until Justinian that this whole arrangement and idea of the Germans continuing Rome all broke down. Justinian's quest to restore Rome ironically destroyed the idea of Rome. This is where we see the definitive and absolute shift away from the idea of Rome to Rome being a nation of a specific religion (perhaps the Slavs converting to the faith of Rome is why they were assimilated at that later date?). Anyways, Justinian's viewpoint was informed from his upbringing in the countryside where religion was the paramount identity of the state and absolutely rejected the Arianism faith of the Germans. Justinian’s reign marks the moment when imperial resources were deployed aggressively to impose doctrinal unity. (Ascension of Justinian being often the definitive start of the Medieval Age, where religion would be the most important and this would not generally change until the Peace of Westphalia). This was in strong contrast to the urban intellectual's views of what meant to be Roman, where religion didn't necessarily determine if you were Roman. Basically, his court was in favor of practical pluralism, while Justinian wanted imperial absolutism. With Justinian's rejection of practical pluralism, this would start the shift of power away from the Emperor of Constantinople to the Pope in Rome as Pope would increasingly have much more sway over the Germans compared to the Roman Emperor (many other factors led to this shift too, but I view this as the start).
So, what does this all mean? Even with Romulus being disposed, Western Rome did not "fall" in the views of Constantinople. It wouldn't be until Justinian's reign that view would shift. I think that Justinian could have increased Rome's power and influence over the Germans with minimal conquests (now this may be hard due to Justinian's background, but let's assume he listens to his court).
This would be my ideal conquest list: Take all the lands of the Vandals (including Sardinia and Corsica), Sicily (and maybe Naples) from the Ostrogoths, Balearic islands and Gibraltar from the Visigoths, and maybe reclaim Ceuta from whomever. Then I would stop. I would have secured the breadbaskets of the Mediterranean Sea (and also reclaim Crimea to control the grain out of Ukraine, thereby greatly supplementing the grain supply). This would have given Justinian absolute leverage over the Germans with so much food, Germans trade with him for his grain in exchange for raw resources and concessions. Keep the tribute system going and expand it. I'll get into that later.
List form:
Justinian’s limited objectives:
• Vandal Africa (incl. Sardinia & Corsica)
• Sicily (+ Naples as forward base)
• Balearic Isles & Gibraltar (from Visigoths)
• Ceuta & Bosporus / Crimea grain corridor
[Basically, Justinian chooses this limited naval policing in Africa/Sicily and foregoes the protracted Gothic War; the treasury therefore retains c. 3‑4 million solidi and 100 000 veteran troops over twenty years that would have served him very well in the East].
To smooth things over diplomatically with the Ostrogoths and Visigoths, proclaim that the Vandals were punished for wanton destruction of Rome over a century ago (Vandals were the death blow of the Romans; the word vandalism comes from the word Vandals, to give you an idea of their lust for wanton destruction compared to the other Germans) and for their persecution of Nicene Christians but in his grace, Justinian would spare the Visigoths and Ostrogoths.
The taking of minimal land from the Ostrogoths and Visigoths should not ruffle their feathers too much and they'll be intimidated by Belisarius' glorious conquest of the aforementioned regions. Intimidation works as long you don't get mired in an endless war...like historical Justinian's mistake, but here we avoid that fate and take minimal land from them. Looking at Justinian's conquest in my version we see that he can maintain this with a strong navy and marine force. While he can keep his armies focused on the Sassanids. This allows his army to not get overstretched and allow him to invest in a navy and marine force that can rapidly support his newly acquired territories at a high cost efficiency. We can look at the domino effect of this where the Arab conquests of Roman territories fail or much smaller in scale, which we can get into later.
Let's look back at further concessions from the Visigoths and Ostrogoths (and other Germanic tribes) he can get. This requires playing up that they are still Romans and he is their "Emperor". This can be slowly done over time where if they need food assistance b/c of bad harvests or they need money to fight rebellious lords or finance war, Justinian can grant food aid from his overflowing, multiple breadbaskets and he'll have the money since his treasury is not squandered. A concession can be that they adopt the Nicene faith (or take baby steps to it over time, such as having them appoint a local bishop of the Nicene faith), or they send forces to Justinian to help with his wars against the Sassanids, etc.
Overtime, just as the East was doing in the West before it "fell", Eastern merchants can resume their investments in manufactories in the West where labor and land was cheap, this is due to increasing stability in the West and respect for the Emperor (perhaps these manufactories would be off limits in the wars b/w the Germans, kinda like how churches were to an extent.). Lastly, Justinian and future Emperors could grant titles to the Germanic leaders, like Keeper of the Roman faith in Hispania to the Visigoths, etc in exchange for concessions. All this will deepen diplomatic, financial, religious, and military ties with the West and East.
These sort of ties would have come really in handy when the Arabs begin their conquests. Emperor Heraclius could call on the Germans to aid in the defense of Rome as "fellow Romans". This most certainly would have blocked Arab conquests of the Romans or keep losses to minimal that could be reversed. The Arabs may have focused their attentions further East of Persia if facing such united resistance from the Romans.
Additional note: Lastly, consider that over taxation for the Gothic War physically weakened the Roman people as this lead to increasing malnutrition which weakened their immune systems and made them more susceptible to the plague. Also, consider if the treasury was not depleted, he could have helped mitigate the plague too.
Speculation time:
Given this timeline, historians would not see the fall of the West, but an evolution of the West.
Some possible domino effects of all on the grand scheme of things: Without the seats of the archbishops of the Nicene faith (Christian) faith being intact from Arab conquests, religious authority is never separated from the Emperor. Since the Germans are acknowledged as Romans of the West, they may possibly get involved in conflicts involving claims on the throne of Constantinople (though hard to predict when this may occur). On the other hand, German leaders or nobles may will their land to the Emperor of Constantinople (just as was historically done with the Church) and through this mechanism Constantinople regains direct control over lands in the West over time. Eastern Rome may even withstand the incursion of Slavs in the 8th-9th century and even the Turks later on with the Western halve being even more so integrated back with the East by that point.
With that all said, would Rome regain England with this setup, I think that would be a stretch, though it could be possible that once the Visigoths and Ostrogoths were reintegrated back into Rome over the centuries, this system could spread to Franks and Burgundians and eventually into England. The ideal situation would be this ever tightening system of tribute would expand across Europe as more and more tribes seek to be under the blessing of Roman rule, which would at a certain point make Rome seem eternal.
All this would greatly soften the "Dark Ages" at least in the Mediterranean region. Though, I do wonder with the weakened influence of the Church, how would that play out in the British Isles, Scandinavia, Germania, Poland, etc. In our timeline, Irish monks preserved Roman knowledge and kept the "light" on. Would Northern and Eastern Europe be less developed in this timeline during the Middle Ages without the Church? Those regions may have stronger presence of paganism. How far would Christianity spread, not just to the nobles but the commoners without the Church as we know it? The Mediterranean region would continue being the center of power, which may mean that Europe does not end up colonizing the world (no Ottomans). And if England is less developed, do we even have an industrial revolution? Does Poland and Hungary developed enough to withstand the Mongols? It all depends on much the Romans can make use of this "system" I envisioned over the centuries, millennia. So many dominos to consider.
Justinian, in his quest to restore Rome in his vision of a pure Rome, ultimately was folly and doomed the Romans. All because he wanted the city of Rome back.
TLDR: Had Constantinople continued the fifth‑century practice of recognizing Gothic kings as imperial lieutenants—while limiting direct reconquest to Africa and key islands—the Byzantine state might have entered the 540s with a healthier treasury and intact Italian taxation. That reserve, in turn, could have funded larger frontier armies when plague, Persians, and Arabs struck in succession. The result would not be a Roman ‘renaissance’ in the modern sense, but a stronger, more federated Mediterranean commonwealth in which Gothic, Roman, and even Slavic elites all competed for imperial favor; whereas, the reconquest‑and‑overstretch model of historical Justinian failed.