I think the critical questions are (a) whether Rondo's good enough to be a PG on a championship team, and (b) what kinds of players he needs around him.
I'm not sure why a) is a "critical question." Is Rondo good enough to be the point guard on a championship team? Yes. Other point guards who were good enough: Derek Fisher, Mario Chalmers, Avery Johnson, Antonio Daniels, Steve Kerr . . . etc.
It's fairly obvious that simply being good enough to be a starter on a championship team doesn't mean much. That's a relatively low bar.
What does mean a lot, however, is being good enough to be the focal point, or at least a core piece, of a title contender. That gets to your second question. What kind of teammates does Rondo need to have in order for his team to be a title contender?
Personally, I think there are probably a number of different ways that you can build a team around Rondo. It doesn't have to emulate the Big 3 setup, and it doesn't have to be a bunch of young athletes who can run.
I do think you'd need to give Rondo at least two really high level scoring options who can command extra attention from the opposing defense. I'd say also that the supporting cast for Rondo's team needs to feature a number of floor spacers to give him plenty of room to operate. Danny has done a pretty good job of assembling the right kind of players for the supporting cast, so far. But the two elite scoring options are still missing (with all respect to Jared Sullinger), and those are the hardest to acquire.
The most important thing is that I think you need at least one player who is significantly better than Rondo (i.e. a top 10 talent) and another who is on a similar level (i.e. in the conversation for top 5 at his position).
After the latter paragraph is where BBallTim will come in and mention that Rondo has been the best player in playoff series that featured LeBron James and Derrick Rose, and the best player in individual games that featured plenty of other really talented players. That's true. I think it's indisputable that, at least prior to the knee injury, Rondo was capable of taking over a playoff game, or even a playoff series.
I don't think you can expect him to carry your team to a high playoff seed and win you multiple playoff series, though. He's a really nice player, but he's not that kind of player.
What does all of this mean, if you accept it, for the Trade Rondo / Don't Trade Rondo debate? I think the answer depends in part on your rebuild philosophy. Is it acceptable to put together a group that can compete for a middle seed and perhaps win a playoff series while lying in wait for an opportunity to add a piece to vault the team to the next level? Or is it important to be more patient and assemble a young talent base with a higher potential ceiling?
I don't think it often makes much sense to sell low on a player, least of all a player with Rondo's resume. That said, Rondo is a year and change away from entering free agency, and he's not that far off from his 30th birthday.
It's fair to wonder if Danny can put this team in a position to be more than just a decent playoff squad in just a couple of years. A 2-3 year rebuild is not unheard of, but it typically takes teams a good deal longer than that to really get back to the top.