It's not necessarily delaying rebuilding.
Option 1: renounce pierce and allen, start rebuilding, and sign who exactly? The second tier FAs are getting widely overpayed -- would you really prefer to have Rudy Gay (if we could have gotten him) at $80 mil over 5 years? That's the best case scenario for rebuilding, and it's not that good.
Option 2: resign pierce, allen, go for it in 2010-11 with the mle. Now if they look worse and are visibly aging, you trade Garnett's expiring in 2011-12, Allen's expiring in 12-13, and potentially Pierce's expiring in 13-14. They could still be competitive in any of those years, depending on who they can bring back, which depends on things which are currently unforseeable, like which teams want to unload superstars and rebuild. But it certainly seems plausible that someone will value the cap space over the players, and that we can rebuild on the fly.