Having Cena win this match doesn't really do anything for him or WWE. He doesn't need to be elevated. Even without the strap, he was the Main Event. Winning and "getting the torch passed" just elevates him further. On the other hand, losing that match opens up a whole new batch of options. Losing streak? That would make him a more sympathetic figure, and could serve to get his skeptics behind him again. Heel turn? I know I got goosebumps when he came out in the Cavs jersey with the chain. I know that guy won't be back, but Cena could be phenomenal as an unstoppable monster heel. Even if neither comes to pass, wrestling is just more interesting when you don't know 100% of the time that a guy isn't going to lose clean.
I don't see how this does anything for The Rock, either. He's doesn't really wrestle any more. He just does these special appearances. Unless this pushes Cena to change his attitude or give him a heel turn, then there was really no point in him losing.