Well, them and Magneto, who as per usual is willing to take more drastic measures.
Just like Patrick Stewart’s Professor X, James McAvoy’s version took measures to keep Jean from being taken over by the Phoenix Force when she was young.
Oh, and let’s not forget Jessica Chastain’s unnamed character, an alien shapeshifter who wants to harness Jean’s incredible powers so that her species can rule the galaxy.
Presumably this is what causes Jean to lose control, leaving it up to both her teammates (including Cyclops, who looks to now be romantically involved with Jean) to stop her.