I would submit that Kerry's only out is suicide. It's game over. If he has any honor (which he has yet to express any time in his life), that is his only option.
There appear to be two paths ahead of him. Either outright defeat in the general election, or stepping down for - whoever.
If the latter, then it becomes - who picks? They've already had their convention. Edwards moving up to the number one spot would be no improvement. So?