Wallace is too bad at too much money to retain if we have any hopes of being really good in the next two years.
It's not an unbearable burden if he is the only bad contract on the roster.
It will require an extraordinary amount of precision (or luxury tax spending) to be really good if you are burning $10 mil on Wallace. Dallas managed to do it with Caron Butler on the fritz, but it makes contention much more difficult.
Even if Ainge were to magically remove Wallace's contract from the Celtics' books for next season, he wouldn't gain any meaningful cap space, unless he were to renounce Bird Rights for Bradley, Humphries, and Bayless, and also the rights to the MLE.
If Wyc and Co. said that they wouldn't pay luxury tax again, that would be one thing. But they've repeatedly demonstrated that they're willing to spend to compete.