You call the Harry Potter movies quality and faithful? On what basis? Sure, the movies are very decent works at best and sure, they have high production values and for the MOST part, they are faithful to the ONE book they're adapting. So, unless X-Men was novelized into a handful of progressive books with one story, one set of characters, and one main protagonist (people already didn't like the idea of Wolverine being the main character), among so many other factors, you might be onto something, but you can't compare them that way otherwise because they're not the same.
Heck, your analogy towards X-Men is similar to what I feel about the Harry Potter movies. They don't have an incredible amount of depth (unless you look at the movies as a whole) and pretty much every movie felt the same, but that's the point! They're supposed to feel connected.
I will agree on you about the depth in X-Men movies, but surely that's not what's guaranteed to make a better movie. Even in the Harry Potter films, depth has to be divided as the story requires it. You shouldn't force development into a character if it isn't necessary or at least entertaining.
Look. The problem with the X-Men movies started at #1--the cast. The cast was too big, too expensive, and it kept growing. They've been complaining about that since X2.
I'm sure they'll reboot the franchise eventually, but this would be an arbitrary call to do so now. I mean, these movies are still doing a lot better critically and financially than the Fantastic Four films.
So, I agree with you....partly. Harry Potter just isn't a good example, but I get what you mean.