Why would Apocalypse not be sufficient for that?
Having a villain that can destroy civilization and somehow, all the heroes make it out okay is unrealistic. Xavier being kidnapped and possibly tortured is bad, but not "destroy mankind" bad. Especially since he survives Apocalypse.
We've seen Future Xavier for all of 2-3 minutes, so we have no idea how things work out with Moira.
Are you talking about the end of DOFP? That's true, her not being at the school that day doesn't mean she's dead. But it also confirms that she is one of the few in the X-men group whose fate isn't confirmed. We know Jean, Scott, and Storm live, and from the trailers, I'm pretty sure Raven, Quicksilver and Hank live, too. The pool of people who can die is pretty small.
Someone has to die during the final battle, and Moira makes sense to her emotional ties she has to Charles. I'm not saying I like it--I really like Moira. It just makes sense from an storytelling standpoint.
This I really disagree with. The whole point of the series is to disprove Apocalypse's philosophy; the idea that Moira is not "strong" and will die as a result goes against that.
I'm not agreeing with Apocalypse--from his view, all humans and most mutants are weak and should be exterminated. Moira is a very strong woman (she's not a mutant but is going into battle anyway? That's a badass right there), but again, if all the heroes survive, it undermines how dangerous the villain actually is.
We haven't seen much footage of that fight at all, so I'd rate that as not terribly significant.
Based off the trailers, we've seen enough to determine that Moira's fate is one of few that's uncertain.