I think the clearest mistake Ainge has made recently was not trading Rozier when he had the chance. But that’s a mistake on the periphery - not something driving the rebuild we’re seeing now.
If you really want to give Ainge a hard time, you can add not trading for Kawhi to the list. He presumably passed because 1) he thought Kawhi was likely to leave after a year; 2) he didn’t want to trade Jaylen for a 1-year rental; 3) nobody knew whether Kawhi was healthy. And we’re seeing why he was afraid of the 1-year rental problem right now with Kyrie. The players have all the leverage, so trading for a guy who wants to be with the Clippers is fraught. The Raptors did it, won the title, and even now are underdogs to keep Kawhi. (And even then, we don’t know if Jaylen and Smart tops a DeRozan offer for the Spurs).
AD and Paul George were never possibilities, either due to the CBA or due to needing to keep cap space open for Hayward. Complaining about those non-moves is just nonsense.
The Celtics are in rough shape now. It sucks, and that’s about it. Getting angry at Ainge about it would be asinine given the specifics of how things went down. I’m pretty pleased there’s no “Ainge hot seat” thread here right now however. It’s just the reality of the NBA - it’s hard to win, and there’s no blueprint. Sometimes you do everything right, and it just doesn’t work out.