It doesn't matter why you fumble, only that you do. As for the NFL cleaning that up, that is not guaranteed. Tiki Barber had a very good career, but it took him 7 or 8 years to solve his fumbling problem. One of UM's best backs ever was Chuck Foreman. Chuck never solved his fumbling problem. Having said that, I don't think Duke's problem is nearly as bad as those two guys.
The kid gains nothing by staying, he adds more mileage to his legs and delays getting to that 2nd contract by a year. Kid has been the heart and sole of this offense, it's time for him to take his skills to the pros.
Except the opportunity to improve upon his legacy and try one last time to bring Miami back, play more football with his close friends, earn his degree, get stronger/faster/better and improve his draft stock, be a college kid for one more year, etc.
The reason why Duke should go to the NFL is obvious -- get money now in case he gets injured. Just don't pretend like there aren't plenty of reasons why Duke should stay. For all we know, those reasons may mean a lot more to Duke than jumping ship to get paid.
Duke is a good kid who truly bleeds orange and green. It's not unfathomable that he would want to give it one last chance, especially if Miami takes out an insurance policy on those million dollar legs.
Edit: I'm not saying he should stay or he should go. I'd love if he stays and I'd be happy for him if he goes. Just playing devil's advocate.