When I was ten, and read the last of the Narnia books I wept because I had gone through all the books and none of them would ever be new to me again. It wasn't about the Wow finish (in fact I think I was rather irritated that poor Susan Pevensey was locked out of heaven just because she'd grown up); it was the sort of sense of loss you have when a friend moves away. You might visit that friend or revisit the characters in the book, but the relationship changes, and what you get from the relationship may not be what you prized. An intangible sense of comfort and support is being taken away--comfort and support you very likely didn't realize had become important to you.
I'm not saying this feeling relates to literary quality; it's more about finding a universe that makes sense to you however harrowing the events that take place in it might be. So if the Harry Potter books are getting darker, or Jane Eyre has had to leave Thornfield, or Buffy is dead, for God's sake, there's still the feeling of knowing the rules and believing in your companions. It's comfort reading, and I say it's a force for good.