Meh, regardless on whether or not she HAD to play Ripley, she made the decision to have Ripley killed off to put an end to Ellen Ripley as a character. Looking briefly, one cite was a little more charitable with Weaver explaining it less “Playing Ripley is a chore” and more “Continuing to have Ripley as a character lessens the impact of the stories”:
There’s only so much bad luck that a person can have. For her to continue to wake up and confront the alien and resolve the situation, then go back to sleep and wake up to yet another situation — to me, it’s a burden on the whole science-fiction premise of the alien.
If this was her motive then it still makes sense for her to want to have the character “permanently retired” than potentially recast with another actor.