I'm of the opinion that the teen novel has stunted literature in general.
As a kid beginning to read in the 70's I jumped from what were clearly kids books to Lord of the Rings (pure chance -- I liked the cover). There wasn't that watered down in between. It was a jump to real books with real consequences (spoilers: Boromir dies).
I've witnessed the rise of the teen section and seen how kids -- who are reading less in general -- never leave it.
It feels like the fantasy adventure lends itself a lot more to these teen novels and has a knock on effect into the mainstream. I for one could do without anther book about someone born to be a prince(ss) up against the evil realm who can't choose their way out of their romantic triangle.
I'm not knocking anyone's choices. There are more books already than I'll ever read. But it should would be a blast to get another Dune out of nowhere.
I would posit that the issue lies not with YA novels or fantasy in general, but with the continuously declining sense of agency, opportunity, and self-realization among teens and adults. There are deep-seated needs that reality can increasingly no longer meet, which results in rising dependency on simplistic escapism.
I remeber reading tons of junk and pulp as a teen decades ago. If anything, the teen section is more complex then pure hero fantasies and repetitive scary stories that used to make the bulk of reading. Yes, lord of the rings was popular before and now. But the stuff people read en masse was not lord of the rings and certainly not some kind of smart complex stuff.
What used to be read a lot was literary equivalent of a sitcom or slash fantasy.