I agree. I keep thinking ChatGPT's conversational abilities are massively oversold. Perhaps our expectations of human communication have been ground down over the years by 140-char discourse and 15 second videos.
Challenging writing has been iteratively squeezed out of books aimed at young readers. The goal of addressing as large a market as possible means every publisher wants all their authors targeting exactly where kids are, or a bit under, to maximize appeal. A couple decades of that pressure means "where kids are" keeps becoming a lower and lower target, because none of their books are challenging them anymore.
Options outside of YA are dwindling because YA, romance/porn, and true crime / mystery / crime-thriller (all aiming at ever-lower reading levels with each passing year) are the only things people actually buy anymore, in large enough numbers to be worth the effort. Other genres simply can't support very many authors these days. Sci-fi and fantasy are hanging on mostly by shifting more heavily toward YA (and sometimes romance), as you've observed.
Friendship ended with Amazon. Now Project Gutenberg and Archive.org's magazine rack are my best friends.
I'm not disagreeing with your sentiment. I love richly written, complex writing that can take a moment to digest, but, let's be honest here, it isn't just AI that has destroyed the written word: the internet, smart phones, and cute emoji have already done an exemplary job of that.
I cannot find any more fantasy literature that won't make me puke a little bit in my mouth every time I try to read it. Granted it all seems to fall under the grotesque umbrella known as YA so perhaps it cannot be helped, but where or where are the authors who wanted to expand the minds of their young readers? I cannot find them anywhere.
When did you last see any sort of interesting grammatical structure in a sentence? They are bygones. And it depresses me.