What if Starlight had rejected Twilight's friendship offer? What if she refused to change her ways and vowed to continue with her revenge on Twilight and her friends?
Starlight Glimmer did not die when she was banished to the sun. She, for some reason, some unfathomable power of will, had chosen to survive the fire. And so, she waited.
Twilight Sparkle and Starlight Glimmer are evenly-matched. Every time Twilight tries to stop Starlight's plan, she fails. That means Starlight wins, right?