This has to be at least the 5th time he's died, so by this point, no matter how FOR REALS FOREVER they want this to be, I don't think anyone's ever going to believe it'll last, and I suppose like Nightcrawler's before him in 2nd Coming, it's always foreshadowing This Man Is About To Die when someone who's been a background irrelevance for ages is suddenly pushed into the spotlight mid-crossover, because comics writers seldom, if ever, have the balls to kill off a core member of a team book's ensemble cast that's actually in-use right now, in one of these Big Crossover Events, as much as we usually wish they would do so instead of pulling someone out of limbo to die (having been here since virtually Day One of this board back in 1998, I vividly recall the hyped-up death in early 1999's 'The Magneto War', and so many people agreeing it should be Colossus, who was just....there, doing nothing [as usual, no less], only for them to bring back Joseph to be The One Who Dies).
And I suppose given that no-one's had any solid plans or ideas of what to do with him in years, so when he inevitably does return, it should (in theory at least) be because someone really wants to bring him back and use him and doesn't believe he's obsolete or unneccessary, as more writers than not seem to these days. In a way for superheroes, unless you're ultra-obscure or unpopular, being dead can be less-worse than being in limbo, because more people will remember you and actively want you back.
But anyways, taking into account that these things shall pass, and it will inevitably be reversed (well, unless the world really is ending in December, I guess), so there's no point in getting annoyed by it, for the short-term fallout of this event, they really should be able to milk this one for all the drama it's worth, the X-Men have lost their founder, their mentor and inspiration, and at the hands of their leader, no less. Does Cyclops get a "Phoenix Made Me Do It" excuse, and it's all about him living with what he did, or is this the point of no return and he's following Bishop into the realms of full-on bad guy? And AvX really needs to not end with the Phoenix power creating any new mutants at all, because doing that would be some sort of sick vindication and validation of the entire crazy train Scott's been on this whole time, and on some level they'd be able to say it was all worth it in the end, even this.
Also, for those suggesting it below, Xavier can't "come back as Onslaught", since Onslaught is already Onslaught, and is still around. Looks like we'll be waiting even long to see him finally fight his two "fathers", though. Xavier pulling a Shadow King and surviving in astral form, now that's certainly plausible. As is the X3 Movie option of him escaping the moment of death by carjacking someone else's body.