Yeah, but . . .
they're all going to die (except Jack) in this original timeline, ain't they?
It just didn't happen to all of them at once.
What this proved to me, though, was that it was JIN who was the actual candidate and not Sun.
The candidates can't kill themselves except to sacrifice themselves for the ultimate candidate -- once Sayid said, "it's going to be you, Jack" then he could take the bomb and kill himself in the process. Once Jin told Jack to go, he could sacrifice his own life to save Jack (even though it also meant dying along with Sun). Sun was only sacrificing herself for Jin, not Jack. So, for me, she wasn't the ultimate candidate; it was Jin.