Pushing for "challenging" reading strikes me as absurd.
People already don't read as much as they ought to. Forcing kids in school to read thick, complicated, "challenging" works is going to accomplish exactly one thing: making them hate to read.
You want a challenging read? Try slogging through Moby Dick or War and Peace. All you need for a piece to be challenging is for it to be a chore to get through.
Sometimes a book can be worth reading to a person despite being difficult - I'm personally reading through Lawrence of Arabia's Seven Pillars of Wisdom because I'm personally fascinated by the historical context of the piece. But if I weren't already motivated to read it despite the challenging nature of the piece? I'd never have made it past the first chapter.
It's not a book you can pick up and breeze through - it's super heavy duty reading, placing a huge burden on the reader of keeping many complicated details in their heads, and written in a very particular style no longer used, by a very complicated man who by today's standards held some very unpleasant beliefs. There's a lot of sifting and sorting and analyzing and inferring you have to do to properly understand the text, and especially to overlook the aspects of self serving British Nationalism and Colonial Imperialism, all while fitting it into a large historical context.
I'm certainly not reading it just to have something to read. You'd have to be crazy to read something like this just for fun. It's very "challenging".
YA fiction does literature a great service by being not very challenging, and by being relatable to young folks. It exists pretty much entirely to give kids more reasons to enjoy reading. Not everyone grows up willingly reading Tolkien and Dumas like some of the folks I know personally.
There's nothing wrong with making books approachable, especially in an age when so many other less educational activities are competing for people's interest. I'd rather have kids reading Harry Potter and The Hunger Games than not reading at all.