I definitely share a lot of the same worries. I think Rondo still has a long way to go in his game to reach that true superstar PG level to carry a team. I think without a great supporting cast, unless he can really improve with his shot (which I have certainly not ruled out, but have not seen yet), he will become very average.
I also worry about him as a guy who relies so much on athleticism, that if he ever does get a serious injury, I am not sure he will be able to recover from it and still be effective.
This is why I think it is so imperative that the C's do everything they can to surround him with other stars. He is not going to be the type of player you can just surround with good role players, and hope he brings you to the promised land. With other stars around him, he has a special skillset that will make everyone better (including himself).