Yes I remember that story by Mike Carey, one of my favorites from Legion and his Age of X story
. I agree with you he could have teleported him to Elixir but I think David always had a problem in the healing aspect of his habilities, for example, he´s also a world class telepath but it took him years to wake up from his coma, not even Charles could bring him back from that and it took him some time to reunite the scattered parts of his personality, especially after Nemesis messed with them so it may just be me but I think Legion journey implies that he needs to learn to heal himself so he is able to heal others and interact with them in a more positive way in which he´s supporting, helping, healing, fighthing etc because he wasn´t able to do any of that before, his past crisis has convinced him he messes up all the time and when he messes up universes can end, he knows for a fact this is why his father is terrified of him and he gets it but it still hurts, so Magneto giving him his trust in something so important to Erik, like the survival of Arakko Mutants, was the positive boost he needed to act and use his powers well.
I think Spurrier has been leading the character to this point. It also serves him as a personal pathos for his character, because
while he´s extremely powerful that wasn´t enough to revive his Mother or stop Mags from dying, just like Magneto wasn´t able to save Anya, despite his newly discovered powers. I think the fact both Gabrielle and Erik as friends/family of his father and his mother/parental figure means their deaths affect Legion more deeply.