Well, did do it, or at least tried something _like_ it with the original cast, and it was awful. Bryan Singer's original idea was to do the Phoenix story line across 2 movies, (it was hinted at and started in X2), but he bailed as director, and they kind of just munged up the story into a single movie without much regard to the source material.
While I don't think it's a great movie, the setup/handling - without getting too spoiler-y really hits the right notes in X-Men: Apocalypse. I actually like Last Stand and DoFP quite a bit, the Fassbender/McAvoy roles are really spot on, some of the other cast are a little flat, but it's tonally pretty good, and the Quicksilver segments are almost worth the price of admission.
FWIW, I have a long run of John Byrne X-men up in storage, including all the Phoenix material, beautiful mint, 1st editions
[doublepost=1518456411][/doublepost]Speaking of things that _are_ part of the MCU ... ;D
Anyone else stoked for Black Panther?