Once upon a time, there was a man.
A man - not the Doctor now, how can he be for a universe where he doesn't exist, doesn't belong anymore.
The perfect universe, where the cracks never even opened, never ever were.
He's taking a chance. A gamble he might not win.
But it's one he has to take.
Well, at least this way, he can be sure that he won't/hasn't/never will screw up anyone's life again.
The cracks close.
Once upon a time, a man stole a magic box and ran away.
And they lived happily ever after.