SPOILERS:
That was never going to be the ending of the film. The original ending had her escaping and discovering the entire city had been destroyed. So tying it into the Cloververse was just a small extra step.
I actually would love to see a cut of the movie with that ending. I’m assuming it was never filmed though. I think in a way though the theatrical ending was better since it gave here the opportunity to take control of her own life. Something I was never quite sure of was how long she was actually inside of the bomb shelter.