You can make a really ugly, low quality change to a language, wait for a new generation to grow up with it and it will now be accepted as perfectly fine. There isn’t any objective notion of quality here

That doesn’t imply it’s perfectly fine, it might just mean that the arbiters have lost their ability to detect quality. Which is exactly what I think has happened.

What you call signs of quality are cultural signifiers. My native language is French, a language that has an actual gatekeeping administration (English doesn’t). The french I grew up with(not in the country of France) may be considered lower quality by some people because they aren’t used to it, but really what they mean is that I express a different set of cultural signifiers they are used to.

Unsurprisingly signs of language qualities have a tendency to reinforce the language spoken by people in power.

I don’t think those are the same things. I had in mind an example more like this:

A town is full of carpenters that make furniture. They understand the variety and quality of various woods, from oak to ash to ebony. These carpenters can easily discern the quality difference between one wood and another.

Over time, the carpenters die out and are replaced by people that can’t tell the difference. To them, an IKEA table made of compressed wood is the same as a handmade table made of high-quality wood. Ergo they have no ability to discern the quality difference and think they are all the same: wooden tables.

In terms of language: if language is merely becoming more simple and following its own rules less, then that seems like an analogous situation. It’s not simply becoming something else, it’s becoming dumber, less complex, less adherent to the rules that previously defined quality. It’s not doing this as a consequence of pursuing new levels of quality, but merely because the previous ones are decaying. I don’t think comparing two languages like French and English together is quite the same thing.

I would have considered the use of the Ad populum falacy more for use in terms of testable facts rather than opinions.

Is quality of language and taste opinion or fact? I could see the debate being vigorous on that one.

It’s a big debate indeed and I don’t want to get into it here, but I think I come down on the side of, “some standards are not purely popularity contests, but are based on other things.” There are a lot of reasons I think this way, but even if someone doesn’t agree, I do think a purely consequentialist approach is illustrative.

Would we have better food if the top chefs in the world designed our meals, or if the entire population voted on them? For some topics (including the arts) I think a purely subjective approach has worse outcomes.