I have a feeling, if Sunset stayed, she would have held the Element of Loyalty. From what little we see, we know Celestia cared at great deal for her. And seeing how Twilight fawns over her, it's safe to guess Sunset would have been just as devoted. Something big happened between them, something to shake, then break Sunset's loyalty. What more sad is she was a better student. She liked going out an experimenting (latest Equestria Girls commercial), instead of believing everything someone else says because it's in a book.

Sunset Shimmer would have been a far greater pony than Twilight.