re: "If you come back next year, I'll win you a championship."

It all depends in the expected "draft" ranking and whether it can be improved by staying in school or not.

After a certain "ranking" and the money that comes with it...if any player that needs money stays in school, they are crazy... one injury and it is all over.

Those who do not need money like Andrew Luck and (I am assuming) Matt Barkley are the exception.