As for the First Class comparison, it really doesn't hold up.

The main trilogy was retconned and written out of continuity for the sake of a clean slate and a new beginning where they can go in any direction. But with the happy ending seen at the end of Days Of Future Past, it no longer can go in any direction. It has to end up at that one point.

As far as First Class goes... The intention of that movie was completely different. The intention of that movie was to show us the "how", not the "what". The send of consequence came not from what happened, but HOW it happened. Magneto being responsible for crippling Xavier changes how I look at the main trilogy. It changed the dynamic of the relationship. We knew there'd be a falling out, the drama was seeing how it happened.

Things like seeing Beast and his relationship to Xavier in DOFP change the dynamic of X-Men: The Last Stand. It adds so much more to that relationship when Xavier says "you're always welcome here, you're a part of this place". Now we know just how much a part he was.

Before Logan traveled back in time and changed things of course :P

You're making a movie called "Apocalypse" that's supposed to be the highest scale stakes the franchise has seen... But nobody is gonna be in danger of dying. Okay...