I think Morrison applied (or at least, tried to) the same logic used during DPS when Jean survives the D'Bari star supernova and then kills herself with some dusty weapon that came out of nowhere. The latter happened because she obviously let her guard down. And remember that Xorneto gave her a "planetary-scale" stroke.... (whatever that means).
But yes, it was a lame death and it's shame that Marvel hasn't revert it.