Re: What was wrong with the current trilogy?
X-Men was a perfect example at adapting the fantastical comic book adventures of the X-Men into a real life setting without compromising too much, making their uniforms all leather black made more sense than colorful vibrant colored spandex and the performances were done in serious manner.
X-Men 2 was a perfect sequel expanding on the plotlines from the first movie into a more satisfying action pic without shoving every new mutant on screen.
X-Men 3 while still good shouldn't have killed off 3 of the major characters, the story felt rushed with too many characters not getting enough screen time coupled with a shorter runtime for what was intended as a grand epic finale didn't really feel satisfying but having Magneto retain a hint of his powers at the end was a clever idea, too bad further sequels will never expound on that.
A few of the Marvel Superheroes are somewhat niche characters that aren't extremely versatile, thus they don't have a lot of longevity potential. For example, Namor is a water guy, Silver Surfer is a space guy, Dr. Strange is a magic guy, and so on.