I wonder how much of the YA uptick is driven by adults who prefer less-challenging reading. If that's the case it just makes the picture appear even more bleak.

It’s mostly that. Basically the only genres that still sell meaningful numbers are YA (with lots of adult readers, and if we want to count that as its own genre) and romance (99.9% of which isn’t more challenging than average YA, and usually has even less going on as far as ideas and theme—not to knock it, I mean hell, it’s no worse a use of time than tons of other things).

Adult genre fic, even, is dying, and lit-fic has long been in decline and has pretty much just been for a few nerds since roughly the turn of the millennium.

I think the decline of reading is exactly what’s pushed publishers and agents to favor easier and easier books: you have to pursue as much of the market as possible to make money now because the whole market’s not that big, so you can’t afford to exclude readers. That means favoring ever-easier books as readership declines.

The only other route to make a living is aiming straight at film/TV adaptation, which is very hard to break into but a handful of authors have successfully specialized in that. Their books do OK but they’re watched, as it were, way, way more than they’re read.

That kind of issue is very much "eh" for me personally. They're reading. Not everybody's motivated to read "Ulysses" or "War and Peace". Probably a significant percentage move to reading more challenging material with time.

The issue a lot of people are talking about is the decline in reading comprehension, lowered reading scholastic scores, the overall lack of reading, and the consumption of entertainment that requires little reading.

Stuff like the "Treasure Island" (#34), "Oliver Twist" (#56), and "The Hound of the Baskervilles" (#66) still rates rather high up on Young Adult literature sales by Amazon's reported numbers, even if romance and such is the top 5-10. They're reading.