The Phoenix returns and young Jean (as well as adult ghost Jean, Emma, Quintin, Hope, and the Cuckoos) say young Jean either has to beat the phoenix or be possessed. During the battle, adult ghost Jean and young Jean argue with the bird until it reveals its true purpose. The Phoenix is not here to possess young Jean. The Phoenix is here to kill her, so that it can have the real deal (I don't know how the younger would get in the way of resurrecting the older, but maybe it'll be explained later). Since young Jean isn't as strong as our Jean and she's from another universe/timeline, she can't win (And The Phoenix knows it). So The Phoenix straight up kills her and leaves (I assume to resurrect the real deal), leaving adult ghost Jean kneeling under young Jean's falling ashes.
Now if it could just kill the rest of the O5, all will be right with the universe again.