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With a good GM, I think the Celtics chances of building another contender are around 80-85%. Pretty close to a certainty. So long as the rebuild soon and properly (no Joe Dumars rushing the process and wasting cap flexibility on the likes of Ben Gordon and Charlie Villanueva).
I don't know how to gauge this current team's chances at a title yet. I need to see what the rest of the Eastern Conference landscape looks like before being able to put a number on that.
I think an 80% chance is greatly exaggerating our chances of rebuilding a true contender around Rondo, assuming that we're starting with basically a clean slate. I mean, look at Deron Williams. He's better than Rondo, is surrounded by a ton of talent, and yet he's never been in the Finals. There are lots of talent-laden teams that never sniff contender status.
In almost all cases, teams need an elite superstar to win a championship. I'm personally skeptical that Rondo will ever be in quite that class; he's an all-star and a top-five point guard, but I don't think he'll ever be seen as a top-five player in the NBA. That means we'd have to get our superstar via draft, trade, or via free agency.
Unless you land the #1 pick, the draft is a crap shoot. Regarding trades, teams generally don't trade elite superstars, except at the very end of their careers.
That leaves free agency. How many contenders have been built via free agency? The Lakers with Shaq, certainly. Who else? Also, I'm skeptical that players see Boston as a desirable free agent destination. If Boston can offer the most money in free agency, sure, players will play here. However, if Miami or one of the Texas teams can offer the same contract, for instance, would players come here when 1) taxes are higher, 2) the weather is worse and 3) the city of Boston still has an ugly (and inaccurate) stigma with a lot of players?
That doesn't mean that building a contender is impossible, but it's certainly not easy, either. I'd say that the odds are 80% - 85% that we don't make a Finals appearance in the next decade after we begin our rebuilding phase.