We did something kind of radical and crazy in Days of Future Past where we created a new timeline. We sort of broke the established timeline of the movies that had been X1, 2, 3 forward, and we created a new sort of history for the X-Men. Now that weve created that history, we take that to be canon. We take it to be law, and so its really important that we continue to tell those storiesknowing that really the only destination, ultimately, is the little glimpse into the Xavier mansion at the end of Days of Future Past in the future part.
So basically, nothing that happened in any of the X-Men films to date actually matters, for better or worse. Thats a good thing, given how convoluted the timeline was getting, but it does rather take away the impact of the Dark Phoenix saga. Hopefully we might get a better version of that in a future X-Men movie, or a spin-off. Unfortunately, that might be a bit of a spoiler, as it seemingly confirms that anyone seen in that sequence - Xavier, Beast, Cyclops, Jean Grey, Storm - all survive Apocalypse. Source.