I think that people who enjoy reading more complex literature (even though "adult" =/= complex ,and "young adult" =/= not complex) will do so along with enjoying simpler books like titles for younger readers, and people that wouldn't be reading more complex literature might not be reading much at all if they didn't enjoy the stories in young adult books.

and in the end who cares? It's not like you are obligated as an adult to read ~~*~*~complex~**~*~ litreature.