At end of the film Jean gets killed and transforms into phoenix. I they rushed to get that ending when they have not yet explored the X-men world. There are so many characters that needed to be introduced. Many storylines needed to be adapted. Why such a rush to bring a important story so soon. Now this forces the next film maker to bring the Dark phoenix storyline.
shareFrankly with Marvel getting the rights back they need to just not do the Phoenix story. I admit I like it and personally prefer it to be a cosmic event with the Shiar and Star Jammers. Heck, I'd be fine if Marvel just did a movie on them instead of on Phoenix. It could be a good replacement for Guardians of the Galaxy now that the whole cast is leaving because of James Gunn getting fired.
shareI think that's because when these films were coming out, comic book franchises were standalone (no "EU" stuff) and tended not to last that long. Batman and Superman both saw quality dips after installment two. You didn't play long-game, you got your Lex Luthors and Jokers and Dark Phoenixes out right away because making seven movies wasn't part of the plan.
shareSetting up sequel is never a good move. It's a good idea, on paper, but almost never pulled off well enough. BvS was ruined by ridculous amount of time spent into setting up the next movies.
Marvel did it better. They set up the next movies using post-credit snippets. If it works, great. If it not, just disregard it and no one will care. Win win, I say. Doing it in the movie itself, like you said, would only unecessarily constrained the next filmmakers.