It's all hindsight, but it does show that you can never be sure of what these rentals are going to do after they play a year for a team. And we still don't know what Leonard will do, though it looks like he's leaning towards staying with Toronto . Is he motivated by loyalty..or the Championship? Probably both.
I take issue with the statement Ainge didn't trade for AD because of Irving leaving. You can't assume anything about Irving until free agency starts and he signs somewhere else. Before then it's just speculation.
If Ainge had traded for Davis, Irving would have probably been back on a multi year deal. And also Horford. These guys want to win, and AD here would have whetted their appetites. The chances of Irving or Horford re-signing dropped considerably AFTER the Celtics dropped out of the Davis sweepstakes. And the feeling was mutual, as clearly Ainge decided to go in a younger direction. And now there are other free agent options opening up as well.