I thought the prima ballerina was going to dance anyway and destroy her foot and not be able to dance ever again. It was that kind of drama that was missing, and all of a sudden it becomes a murder mystery at the end?? That didn't work for me.
Well, she knows that she can't dance ever again, that's why she quit. I think she also realizes that Claire is going to take her place, so what she did is that she stepped away while she's at the top before having an accident onstage and being forced to quit when she's down. (It might be a strange analogy, but it reminds me of when the show Seinfeld was cancelled when they were on top in the ratings before they started to decline). I did find this ending to be more dramatic for this character, than say, falling down during the performance because her ankle gave up. I thought that would be more expected, and therefore, less dramatic. When she's watching Claire dance in the rehearsal, she realizes her time is up. And that, combined with the scene where she's having sex with her husband looking numb, made it more powerful.
I guess her breaking her ankle would be more predictable, but the show was definitely building up to something like that, the drug use, the injury, the injections, she clearly was on a brink, so her realizing she was past it by looking at the new girl she spent the entire season putting down just didn't feel natural or right to me.