Honestly I don't feel loyalty adds enough to the game. Seeing cities flip in GS live plays is still weird to me. It seems to happen far too often. Are there any real world instances of this happening? Crimea is the main one that comes to mind.
Yes, lots. I've got an old post of mine I've dug up and will paste in here with some examples.
When Persia was growing in the pre-Greek/Persian wars the border nations would have cities flip back and forth as influence waxed and waned. Areas would be conquered or subjugated, then flip right back out and have to be re-conquered or flip back once Persian influence was established.
The first Greco-Persian War saw Greek influence cause revolts on the Western Persian border. This caused a military response to recapture. When Persia invaded Greece, many of the nations/regions along the way flipped without a fight as Persian influence grew stronger in their regions. I mean all of Macedonia flipped to the Persian side, then at the end of the second war once Persian influence left the region those cities flipped right on back (again without conquest).
Alexander's conquests needed constant military intervention as conquered cities flipped (or attempted flipping) back. It wasn't until the successor states became powerful that stability in the region became a thing. Even then cities on the borders of these successor states flipped between the various rivals.
When Rome conquered Spain they didn't invest heavily in the region (in a commercial, develop the cities kind of way) and cities flipped constantly. Areas where there was heavy Roman influence (the southern coast) stayed loyal, but regions in the centre, north and west flipped back and forth with regularity. This went on for a couple of centuries. The Romans would occassionally sweep into places, be all like "ROMA!", think that was everything done and dusted, then it all flipped back again.
Caesar's conquest of Gaul saw many dozens of cities kill their Roman garrisons and flip, requiring a re-conquest. Again, the further from the Roman border the more this happened. It wasn't until these cities were crushed, their populations stripped away and the Romans really set up shop with determination in the region that it became stable (and even then the borders rebelled).
Roman borderlands had flips with regularity across their Empire. Illyria flipped and left Roman control, needing to be retaken. The whole East flipped to Palmyra, then was like "Oops, our bad" and flipped back once the Aurelian decided to exert Roman influence again. Cities in Gaul flipped between Rome and the Gallic Empire once that became a thing. Cities flipped between Eastern and Western Roman Empires. Alexandria flipped (and was duly punished) a few times. The whole of the Roman Border with Persia saw flips back and forth with hilarious regularity. Armenia would bounce between the two Empires, and much of this was political influence rather than military (though military intervention happened, only for the state to go back to flipping from other influences).
Once the Muslims began invading there were flips back and forth between them and the various places they invaded. Spain (again) had flips between nations, as did anywhere there was a border.
During the Crusades cities flipped between the Crusader States and the Muslims.
The Three Kingdom in China period saw cities flip ad nauseam between factions. The locals would run the garrisons out and declare allegiance to someone else.
Even in more modern times. WW2 had uprisings all over the place. The Liberation of Paris was the resistance rising up and kicking out the Germans. The Slovaks rebelled against the Nazis. etc etc etc
It is a very common thing throughout all of history, and it was almost always areas that were on the border where influence was lower and neighbouring factions could provide influence of their own. There is absolutely it can be counted on one hand, it has happened thousands of times throughout history. It's just that these types of things don't get covered unless you've studied it closer it gets glossed over in the general narrative like "Rome conquered the Mediterranean and held it for centuries". The current setup is a solid representation on this. Obviously it doesn't have the intricacies of real world rebellions, uprisings and such but the basics are right. The more peripheral a city the more vulnerable it is. The more influence a neighbour has in the region, the harder it is to keep your peripheral cities in line. If you don't focus enough attention in such an area (ie build more cities, assign you governors etc) then you will have problems.
But this is not due to "pressure" of nearby cities. If we were talking about the loyalty mechanism & pressure of nearby cities, Switzerland would have long ago been absorbed by the EU. In the real world, things work very different.
well Civ4 wasn't perfect either. Back to the Switzerland example, Switzerland isn't potentially gaining cities because of high culture. I would argue Italy has much higher culture (in Civ terms) than Switzerland. Switzerland is coveted because they are a pretty awesome place to live, have a good economy, don't suffer from corruption and crippling debt Italy suffers from. I would live there if I could. The situation is too complex to be modeled in civ terms. Neither population pressure or culture correctly models people wanting to "flip". I actually think unhappiness would be the biggest factor in reality. Unhappiness in the loyalty mechanism is an issue in Civ6, but far less than population pressure. I would rather see negative amenities and low gold production (this should be a loyalty mechanic) have a higher impact than population.
Civilization cannot possibly reflect the complexity of reality when it comes to the loyalty of people and cities to country/ruler/empire. And happiness, corruption, cultural pull all have an impact. Though individually not that much, so simply picking one doesn't work very well. They all play a part, but simply having an unhappy city rise up isn't enough, as there were MANY unhappy cities throughout history who either stayed loyal or cities that were happy but flipped to an empire they really didn't want to simply because of the influence they exerted through their military or power.
I think the existing system is a reasonable representation, because neighbouring cities are a solid indicator of military power and the influence being exerted in other ways (eg Rome held significant influence over its neighbouring regions through trade, political posturing, etc. So the border cities were always the most susceptible to "flipping"). I would love to see it expanded to reflect increased/decreased potency based on factors such as happiness, local military presence, cultural difference between the cities in question etc. Heck I'd even like to see cities (not all, but some whether it is determined by over a certain size, or the top x% largest cities) get a random number generated on creation and that number be added to their chance to flip. Some cities were notorious for rebellions or loyalty for no apparent reason beyond they saw themselves as too good to be part of an Empire, or too close to their Empire to betray them. Alexandria being one of the most notorious, but Antioch has also been prone. And many others. While some have always seemed to stay loyal through thick and thin. Sure it's a RNG but it gives some extra life to the game world.