The Raptors were a Eastern Conference Finals team last season too and we see how that has played out. That really doesn't mean much when you have to get through 23 to get to the Finals and as far as I'm concerned, that won't happen with IT and PG as your main players.
If IT and PG aren't enough to carry them to a championship while Lebron is around or the Warriors out west. Then do the Celtics need to hit the reset button?
Fultz is unlikely to develop into the stud people think he will be for a few seasons. By that point Bradley will be nearly thirty years old if he stays, Horford will be entering his mid-30s, and IT might have gone or regressed. So their key trio are not really on the same timeline, so their choice is to keep them and do what they can in the play offs, or sell them and move onto their young players:
Smart / Fultz
Young / Brown
Brown / Crowder
?
?
If there's no belief that IT and PG or IT and Butler, plus what they've got on their roster can win, then they arguably need to move onto the young talent and hope that they can win, or that when they are ready to win the Cavs and Warriors are not as dominant. Personally I don't see the Celtics accepting that, I think they'll believe another piece like an all star player will give them that title chance.