I am deeply interested in the show's lore. Twilight may be best living pony, but starswirl is best pony of all time. He could totally do the timey-wimey stuff that everypony wants.
this isn't lord of the rings. when you have a symbol of what you will do for the rest of your life on your butt, I think destinies are pretty set in stone in the pony world.
We'll have to agree to disagree on that then. To me, a cutie-mark by itself has little to do with an individual's destiny. That's all I can really say there.