If the team has a good off-season, I see absolutely no reason to believe that they won't be a contender next year. Because of that, I'd supplement the core, rather than blowing it up.
Let's say we rebuild. What are the chances that, as a result of that rebuilding, we're left with a team that has a good a chance of winning as next year's squad?
The thing about rebuilding is that the vast, vast majority of the time, it doesn't pay off. Look at the post-Jordan Bulls for a shining example.
I'm not really suggesting we blow the team up next year. I suppose I worded this: "I'm not here to declare the death of the Celtics as we now know them, but if we can't win next year, should we really be looking to keep the team together?"
Incorrectly. The emphasis should very much be on the if. Beyond 2010, though, I'm not sure what we're doing. You better believe there will be at least 1 eastern superpower born of the 2010 offseason, and the Celtics payroll will still have $40 million invested in KG and Pierce, with whatever we give to Powe/Davis, 4 million to Perk, and Rajon Rondo a restricted free agent. Add in role players, and we're not looking at any real salary cap space even with Ray Allen (I could, admittedly, be wrong on this--I don't really understand this side of the business as much as I'd like).
All I'm saying is that we may have 1 more year left, and after that, I'm worried about this idea of supplementing the core given salary cap issues and how much of a supplement will be needed to really make us contenders not just for the East, but for the title.
There's little glory in just making it to the ECF, or even the finals. Only 1 team really wins every year, and if we don't have a chance to be that one team, why not look towards getting back into that race as soon as we're out of it.