You'd run your team into the ground in 2 seasons if you were a GM in an ill-fated attempt to win now during the height of the Warriors dynasty. Yes, Raptors were successful. They needed insane injury luck to do it (4 of top 6 Warriors got hurt). And they're going to be hot garbage in 2 years. It's fine they're going to be bad, because they won the title. And even more, it was the right move for them, because of where their franchise was. That's not the position the Celtics were/are in.
Gordon was free from an asset perspective and there was basically no cap opportunity cost because they went over the cap with the Kyrie trade and re-signings after that (so Kyrie would have eaten the cap space if they didn't sign someone that summer). Kawhi and AD cost 6-8 years of prime Tatum/Brown plus several 1st round picks for 1 year of play. And Ainge still might have made the Davis trade if Kyrie didn't announce his intention to leave. Remember, they couldn't do the trade until the summer because of the designated player rule, and by that point Kyrie already indicated he was out. Tatum just turned 21 and you have his rights for another 8 years (3 of which are on rookie scale).
They still own 3 first round picks from other teams (plus all their own) over the next 3 years. They still can choose to trade someone. The opportunity to acquire a star has not past, and Tatum himself still has a good chance to become one.