We would lose Brad Stevens. As long as you are cool with that.
I like Brad, but if he would decide to leave because the team determines they are still in rebuilding mode instead of remaining committed to a path toward a first round playoff exit, then I'd say he's not the right coach for this franchise.
I think the main problem I have with the premise of this thread is that you seem to have ingrained the notion that an NBA team can't possibly be in rebuilding mode while at the same time being competitive.
As far as I can see, the absolute genius of where Danny Ainge has this team right now is that he has managed to create a perfect "have your cake and eat it too" scenario.
We've got some solid young prospects, we've got boatloads of good draft picks that don't require "tanking," and we don't have any completely crushing oversized contracts.
We are in a great position to continue to build for the future. Best of all, Danny has accomplished all this while having the team be in a position to keep ownership, our young boy wonder coach, and the fans happy by being competitive while rebuilding.
Why sacrifice wins in the present when we are in such a positive situation?
Frankly, I think Ainge would be foolish to risk upsetting owners, coach, and fans for one or two more prospects who may or may not work out while giving up on hopes of the playoffs.
We are currently mired in a rough patch which is inevitable for a mediocre team like ours. This probably makes it an ideal time for these kinds of "soft tank" ideas to seem appealing and take hold.
It's simply not worth it in the long run, though. I'm sure Danny likes his job as Celtics GM. Right now, he's keeping everyone happy while at the same time having lots of chips at his disposal to keeping building this thing.
As a fan, I'm excited by it. I'll take being a playoff contender while we rebuild. I like cake. I like eating it.