In retrospect, when more talent walks out the door each summer than comes back in the door, it's going to catch up with you eventually.
I don't think there's a single neat narrative to explain all of the things that have gone wrong.
There's Kyrie being weird, Hayward's ankle, the unexpected rise of the Jays, Hayward getting hurt again in the bubble playoffs, bad luck with coin flips on late lottery picks, and so on.
Again in hindsight, Ainge has not done an adequate job of anticipating departures and getting value in return to impending free agents before they jump ship. But you can see the argument for not doing that in each situation.