The logic here is far too simple it's infuriating.
The biggest obstacle preventing the C's from repeating is that it's extremely difficult to win an NBA title against 29 other teams sharing the same goal in the first place, let alone repeat. San Antonio, regardless of good or bad offseasons following titles, has never repeated.
After the first Shaq-Kobe title, the Lakers replaced their third leading scorer, Glen Rice, with Isaiah Rider and Horace Grant (neither of whom broke double digits in scoring) and won the next year. The following year they lost Grant and Rider and brought in Richmond, who gave them a stellar 4.1 ppg...and won the title. After keeping basically the same team the following year, they didn't even make the finals, so the year after they decided to guarantee a championship by bringing in Payton and Malone...lost in the finals.
There is nothing the C's could do to guarantee a title this year. If he brought back the exact same roster, it's extremely likely that we don't even make the finals, and we suddenly look very old and locked into place like Detroit or Miami after their title. No wiggle room, finite ceiling. Then we'd skewer Danny for being short-sighted and overvaluing the championship roster.
Bottom line is it's very rare to win a championship let alone repeat. Ainge could do absolutely nothing to guarantee a return to the finals let alone a victory, and he has to take care of the long-term viability of the franchise.