Rondo is our best player. Can we win a championship where Rondo is the best player? Yes. We were a game away 2 years ago and he was arguably the best guy on the team then (or in a similar role to the one he's in now).
Can he ever be the single driving force that leads us to a title? Probably not. Not a lot of players can. Kobe's done it, Wade's done it, Dirk's done it, and Lebron and Howard got their teams to the championship. There hasn't been a recent point guard whose done it. I think we have to go back to Magic and Isiah to find one.
Rondo's a natural facilitator and not a guy who can put up points night in and night out or close out games. It would be great if he could develop that, but with the tools he has right now, he's the ideal fit for our team. If we could get one of those elite elite guys for Rondo then we should do it. But even trading for another elite PG (Westbrook, Parker, Rose, Nash, Williams, Paul) might not be worth it, let alone a consistently solid player (Lawson, Nelson, Duhon, Jennings, Teague) or a younger guy with upside.