That would be a good reason for why she shouldn't come back.
For what her sisters are concerned, she is dead to them.
Sylvanas gets left with nothing. No subjects, no glory or praise, no family. Everyone she knows and loved either hate her or consider her a disturbing element in their life.
Her only solace is not working beneath anyone, and getting a solid chance at spending her afterlife with Nathanos if she does her job well.
It's probably the best ending her character could get at this point.