I find it amusing and a bit baffling that everyone wants death, death and more death.
When that happened in X3, the fans weren't too happy. In that movie, lest we forget, Scott, Xavier and then Jean died, and probably Juggernaut, Callisto, Psylocke and Arclight were obliterated on Alcatraz when Phoenix went nuts; Rogue and Mystique were cured.
There were people - and still are, on here - who didn't even want Jean to die, despite that being what happens in the comics.
So they have to be careful with these deaths. It could look like a repeat of the X3 deaths, and to see everyone resurrected at the end by the timeline changes could make the audience feel they've been manipulated, because the deaths weren't permanent after all. Also, if main characters die at the start of the film, the audience will know that they probably won't be staying dead because they are a main character.