>>108584709Its a very complicated story, touching a lot of the established Marvel universe, and required the audience to feel connected to her for it to work emotionally.
Jean barely does anything and is not there beyond Scott’s love interest and the first student of Prof X outside of the Phoenix stuff in any of the movies, Scott is barely a character beyond jokes with Wolverine either. No emotional buildup, no emotional payoff.
They didn’t own the rights to a lot of the stuff required for the story, and without establishing that stuff in another movie first you’d have spent half of it familiarizing the audience with who everyone is. Or spent multiple movies introducing and dealing with OC replacements, which would have worked too. Make a Phoenix Trilogy, and have the first mostly about Jean and Scott with some tagalong D-listers.
