Again with historical nonsense. After Alexanders conquests many eastern lands claimed they had Greek culture even though Greek population there was a minority.
Egypt, Seleucia, even Baktria were officially Greek, but ethnicity was not Greek.
That is also 2 000 years before the start of the game, and it's a totally different situation. Those areas had been conquered by a Greek cultured country, and therefore already had some Greek influence. In the game, if you decide to culture change, it's usually because you have conquered some culture and decided that it's better for some reason than the one you already have.
What about Mughals? What about England? What about Yuan? And so on and so on, many nations shifted their culture.
What about them? I don't know much about Mughals or Yuan, but Mughals has a decision to change its primary culture, and Yuan was dead and buried by 1444.
When did England change it's culture? In 1066? Sure, but that was because it was conquered. Later there was some sort of "mutual" assimilation between the population and the ruling class, but you can just give them English culture and it won't really be a problem. If they were to win the 100 years' war, though, that could be early enough that they would remain "French", but how often do they win the war and keep France in a PU? Probably not often enough to do anything about it, but you could give them Normand at the start of the game, and upon losing the war, they change culture to English. It would take about 5 minutes to make that change.
And you need to know what is a culture. Culture it not ethnicity, its tied to social aspects of a group of people.
Ethnic change takes a lot of time. But cultural change does not.
I agree with this, but it would take a lot to change the culture as well. If France were to conquer China, it would never change its culture from French as they already have so much influence and French culture is very different from Chinese. For countries nearby China, like Korea, Manchuria etc. it would make more sense, as those countries already were so influenced by Chinese culture, but culture changes should be extremely rare. Most actual culture changes in the game's time frame have some sort of decision to do it. Another example of a "culture change" with a decision is Prussia, but that is not a real culture change. The culture of the rulers and government didn't change. What changed, was what it meant to be Prussian.
I would say it is not that ahistorical and unrealistic the way it works in game.
It kind of is, though. In particular, it is the part about it being the stated land only (which makes it very easy to exploit). But even that is not enough. If France were to conquer China in 1450 in the game, no other areas conquered (nevermind that it's impossible), they still wouldn't culture change to Chinese even if Chinese now was >50 percent of the stated land, as they still hold on to their homeland and consider French to be their culture. If they then lost France, maybe.