RGIII's future isn't as an Olympian, it's as an NFL QB. With that said, his stock won't get much higher than where it is now. (Heisman winner, 2nd highest rated QB next to Luck, already earned his degree, etc.) The smart thing to do for him and his future career would be to come out now.
As for Luck, same can be said for him. He's the #1 rated QB. By going back, he would only risk his future earnings by either getting hurt, or exposing some sort of flaw we haven't seen yet.
With both of them though, we can all speculate both stances until they finally come out and declare one way or the other.