From Advanced Dark's Stan Lee Thread:
Too much has concluded because Scott, Jean, and Xavier are dead, Rogue is cured, Nightcrawler, Beast, and Angel never stuck around after their inclusions... it's just Storm, Wolverine, Iceman, Kitty Pryde, and Colossus.
Although on it's own, that's not a bad team of X-Men by any right, the core, central characters in this trilogy (even if they all didn't have the biggest roles) were Wolverine, Storm, Cyclops, Jean, and Xavier. And I don't think it's in good storytelling fashion, for an ongoing story that should intertwine with the installments before it, to change the core characters, and all of a sudden turn Iceman, Kitty, and Colossus into the focus of these films, when they never were before.
Okay, sure, Xavier is still alive, and Magneto barely moved a chess piece... but I don't think any of that is solid enough to constitute an "ongoing story arc" vital enough to justify a whole new core of characters.
Plus, unlike X-Men or X2, the ongoing story arcs through the trilogy have been concluded (perhaps not to satisfaction of many people, but concluded none-the-less)
Wolverine's past has been dealt with (even in the comics he never learns the full truth about his past), Iceman and Rogue have "solved" the "problem" of their relationship ( ), the Phoenix has risen, and been defeated.
There may be comic book elements that have yet to be touched upon, but in terms of the movie verse story arcs, there's really nothing left.
I acknowledge that it's possible for an X-Men 4 to happen, and to work. But, I think the odds are against it, considering how X-Men: The Last Stand wrapped things up. I don't neccesarily agree that it was the right direction to take... but it's how things happened, and I think by this point, a 4th film will just be overkill.