Ainge has consistently understood that trades have two components: the exchange of talent, and the exchange of flexibility. Like other good GMs, he understands that gaining flexibility can be as important as gaining talent. (The Telfair/Ratliff move is exhibit A).
The recent trade gives us much more flexibility. We can keep Krstic or trade him, and do the same with Green. Neither player has a long-term contract. We also have a valuable pick that probably can be packaged for a rotation player all by itself.
With Perkins we were looking at a real dilemma: if we had kept Perk and ended up going deep into the playoffs this summer, there would be tremendous pressure to keep Perk, and his price would get bid up by any number of other suitors. We might have ended up boxed into paying him star center money, when he's really just a solid starting-level player.
We won't face that pressure with Krstic and Green.
What's more, this actually gives us considerably more flexibility with Baby. Keeping him is less critical with Green in the fold.
I don't think that this means Ainge won't be able to find trade partners - he just needs to deal with people who value talent more than flexibility. A GM like Presti should value talent right now, since he has two max guys who will be there long-term, and that dramatically reduces how much flexibility he has going forward. There are plenty of other GMs in that position.
Danny understands that right now we are poised to have tremendous flexibility once KG and Ray come off the books, so he didn't want to squander that flexibility now. The Perkins move looks very good in that light.