[saizai]

Another way to express the latter is that if something becomes too popular, upper socioeconomic classes will want to adopt something else, so that they can preserve a class distinction of "higher register" speech. Meanwhile, lower ones will try to adopt the style of the higher register, since it's perceived better. That's an endless cycle.

Same thing for euphemisms, insults, etc. And uniformity was far less common before mass media and centralized schooling…

Linguists have a much more descriptivist way of looking at language use and change.

Nuance is not a feature of education; focus is. E.g. AAVE (AKA Ebonics) is currently considered a lower class register, but it has grammatical distinctions that standard English doesn't (like whether an action is part of a pattern, vs a one off).

ETA: also, an influx of non-native speakers tends to result in borrowing of both words and grammar from their native languages. Not just in the development of creoles. What's "correct" is purely defined by consensus; if there are lots of new speakers, that's of course going to shift towards their preferences.