Manzikert was not that important of a battle, since the Byzantine Empire was already in decline. The result of the battle was loss of gold due to ransom and the opening of the frontier to various Turkish tribes. The opening of the frontier during this time is not very important since the "Franks" sacked Constantinople before the "Turks."
Yeah, Manzikert wasn't that important of a battle, but not for the reasons that you're claiming. The engagement itself didn't do all that much - lost a comparatively competent emperor, a relatively small portion of the army, and a bit of ransom. The civil war after that and the course of those events, combined with the Seljuq irruption into Anatolia, was what ruined the empire. For perspective: Romanos IV got together around 70,000 men during his reign for his various campaigns in the east, yet two decades later it's estimated that Alexios Komnenos only had 20,000 to 30,000 men in his entire army. It's frequently overlooked how badly the bloodletting civil wars and poor management of the post-Manzikert emperors ruined the state.
Of course, even
with that in mind, Manzikert wasn't an empire-breaker by itself because during the 12th century, the empire
did have the ability to recapture Anatolia, at least reclaiming the stable border of the seventh to ninth centuries. Its failure to do so can largely be laid on the heads of the Komnenian emperors, who squandered imperial resources fighting over the relatively unimportant Antioch and making expeditions into
Italy to fight the bloody
Normans for no possible gain while Seljuqs were occupying and solidifying their control of the critical old themata.
I'd say that the Byzantines were in decline only in the sense that they weren't at their 1025 or 540 peaks, not in the sense that it was a
terminal decline. Certainly the emperors of the later Makedonian dynasty were less than competent in many cases (the rather atrocious demobilization of the Armeniakon Thema by that nitwit Konstantinos X being one of the most objectionable things), but since Basileios II, the Empire had actually expanded in the Caucasus, although losing some territory in Italy. It wasn't doomed by any stretch in 1070, or even afterwards, because they'd faced similar and worse setbacks before, to which competent emperors had responded...well, competently. Instead, foolish ones exacerbated a relatively small problem and turned it into the empire-destroying thing that it became.