I feel like you're using the term "rebuilding" rather loosely here. The rebuilding going on right now is what most teams call their normal existence (or better!).
If rebuilding means winning 50 games or more while continuing to build towards being a contender, I don't see how anyone could fault Ainge.
Plus, what's the alternative, blow it all up?
The C's aren't just another franchise tho. That gold ball as my avatar is there for reason. That's what sets the C's apart from almost every other NBA franchise. I'd never be content with a ceiling of the ECF like Toronto is currently experiencing.
This isn't our cieling, though. We've been steadily improving and, even without any big moves, it looks like we'll continue to do so
No GM could go from a teardown to a a championship in less than 5 years without stupid amounts of luck, and it seems like we're one player away from the ECF, which would put us just an injury short of the Finals
Considering that we'll still have good picks coming up, that's a really good spot to be in, whether you're the Celtics, the Lakers, or the Hornets