When Ainge has made good moves, it's always been because he sticks to a simple philosophy: don't overreact to a situation, wait for another team to be more desperate.
This time he completely lost his head and Mark Cuban gave him his own medicine. Stuff like "the market just wasn't there" IMO is what commentators say - smart league execs know circumstances change like the wind. What if Derrick Rose goes down in a month? If one team becomes desperate, Ainge could have got leverage, started a bidding war and got something real that he actually wanted.
Waiting would have made the worst-case scenario slightly worse and the best-case scenario an actual trade instead of a fire sale.
BTW - Mavs taking a risk. Yeah, it's sports. Nothing goes according to plan. It doesn't mean you shouldn't seek fair value on your deals.
If you are going to evaluate this on risk, then you need to do it from both sides. You neglect the risk that Rondo may re-injure himself entirely. That is the worst case scenario. Say he blows out the ACL he just had repared (certainly not out of the realm of realistic possibilities.) And then he still leaves in the summer.
If they had come to the conclusion (as I suspect they had) that Rondo was most likely signing elsewhere, it would behoove them to move as quickly as possible. It's not like the entire league hasn't known for years that he was available.
I find it extremely hard to believe that Ainge panicked or overreacted here, this was an extremely calculated, long term decision that has been in the works for a long time.