As I'm sure you remember the Celtics finished 2nd in the Kevin Durant sweepstakes during the summer of 2016. Strat-O-Matic did some sort of Back To The Future/ A Christmas Carol simulation for NBC Sports Boston to show what type of success the Celtics would have had with Durant.
There are no guarantees in life, but pretty sure the Celtics would have won more than one title with Durant. If there was one moment that was the difference in Durant choosing Boston vs Golden State it would be the Draymond Green suspension in the Finals that is credited with letting the Cavs back in the series.