Given his arm, feet, and what we think we know by now about his smarts, character and work ethic, the only way I can see a healthy Robert Griffin III failing to develop as an NFL passer would be if proves unable to process information in the pocket at elite NFL levels, and get the ball out on time.
That inability is what kept Jason Campbell from ever becoming what his measurables suggested he could be. The lack of quick-twitch delivery mechanics didn't help, but mostly it was that the game never slowed down enough for him to get through progressions and release the ball.
Robert has shown me flashes of that quick-process ability, but he's got a long way to go. Not seeing open guys is a symptom of the game still moving a bit too fast for him. But give him a competent OL pass protecting in front of him, some weapons he can count on, and some time to develop, and I think in 2-3 years we'll all be having a hard time remembering a time we actually entertained the idea that this unusual cat might not be able to play the position.