This is a really good question and I don’t have a good answer. I don’t really understand YA at the moment, and I don’t know if my impressions of the genre over the past ten years are valid any more.
It’s really common to think of the Stormlight Archives as YA, for example, something I don’t think anyone would have done back when Way of Kings first released. The existence of New Adult as pretty much just smut in a contemporary college setting (even when all those trappings are stuck in a fantasy world, like in SJM’ House of Earth and Blood) feels like it’s urging people to draw the upper range of YA upward toward a space between “traditional” YA and adult fiction, especially in speculative subgenres.
I don’t actively write YA, but most speculative fiction I write tends to end up something people would see as YA. Immaturity and illiteracy on my part? Probably.
But also, most adults I know in their 20s-30s, both in person and online, seem to read mostly consume YA stories. That might come through Marvel movies, anime, or actual YA books, but it feels to me like unless I’m targeting specifically mature adults who want something like old school scifi and fantasy (and to my knowledge this isn’t a very large paying audience), the majority of people who will buy indie fiction are looking for YA.
"...the one with whom he so sought to talk has already interceded for him." -The Master and Margarita