Ainge signed/traded for the players on the court. He brought Brad Stevens to Boston. Some of his recent draft selections have been questionable at best. If his acquisitions aren't working, that's on him.
The argument that we shouldn't have added someone at the deadline because we were not in a position to contend was debatable, and I didn't mind much that he didn't make a move (though Nurkic would've been nice). But that logic is flawed if Ainge had any inkling whatsoever that we would get handled in the playoffs as we have. In that scenario, incremental improvement could have been important in luring FAs to Boston. He's going to be hard pressed to garner interest among FAs if Celtics are swept in yet another first round bounce.
In the same vein, the players on his roster are being exposed right now. Crowder, Bradley, Smart, etc. have all probably taken a hit in relative trade value. Would an incremental improvement have taken less of a burden (on either end) off the current players, and in turn, helped maintain their trade value? I'd prefer to have taken that chance, especially given the only realistic FA option is Hayward (who seems better situated in Utah now, anyway).