For a major revamp, you should either do it or not do it. Making it opt-in or even opt-out means every app then has to try to support both different UIs, which is a longterm maintenance cost. Not only for Apple, but the entire ecosystem.

I personally sort of like the liquid glass, but it's also kind of a mess in a lot of edge cases. I feel like it was an interesting idea that didn't really pan out fully and should have been scrapped. It's just too controversial for pure eye-candy.