Factual error: In the car at the beginning of the movie, Jean uses her telekinetic powers to switch the radio from a station playing "By the time I get to Phoenix" by Glen Campbell to Warren Zevon's "Werewolves of London." It's 1975, and Zevon recorded the song only in 1978 (although the song itself had already been written in 1975, other artists played it in live concerts in the Fall of that year). (00:01:30)
Factual error: The space shuttle is spinning due to a stuck thruster. In such a case, the spin would be continually increasing as there is no air resistance. The X-Men shoot out of the thruster and the spin stops. Not factual. They would have needed opposite thrusters to stop it. (00:11:55)
Answer: Your guess is correct. Lawrence stated she was done with the franchise after "X-Men: Apocalypse." She jokingly told Simon Kinberg she would return for this film if he directed it, and to her surprise, he was serious about it and held her to it. Fans weren't very keen on her appearances either, because she was definitely phoning it in for the third film, so that may have factored into killing her off early as well.
Phaneron ★
More than one actress had played Mystique. Just because Jennifer Lawrence did not want to continue playing that role is not a reason the character would be killed off. They could easily recast another notable actress in the part. I would not be surprised if Mystique is miraculously "resurrected" in a future X-Men film with a new actress.
raywest ★
Aside from Deadpool, any new X-Men film would be a complete overhaul of the franchise since the characters will be part of the MCU now. You're not wrong about an actor wanting out to not really be a reason to kill off a character, but it doesn't preclude them from doing so either. They might have decided it wouldn't be worth the hassle of recasting the role with all the negative baggage that would come with it.
Phaneron ★
Here are some candidates I think could do it: Amanda Seyfried, Shailene Woodley, Chloe Grace Moretz. Heck maybe even Milla Jovovich-Anderson should be given an audition, what say you guys?
Rob245