a leads to b leads to inescapably c.
Edith Keeler lives, the nazis win the war, mankind dies out, Starfleet is never founded, the federation neither, and Sisko isn't born... And the prophets are sitting in their temple saying "What the fuck happened here?"
They see the universe from beginning to end all at the same time.
No problem with that.
But there's many universes, and sometimes a universe can be completely reconditioned and facelifted by a little wanton timetraveling and tomfoolery.