Joe isn't an idiot. Brad didn't suddenly forget how to evaluate talent. I wouldn't even blame anything on the new ownership being cheap (yet - too early to tell.) Jaylen Brown doesn't suck. The players aren't lazy. Yes, they want to win.
The league, and specifically the cabal of cheap owners, doesn't want superteams. They want parity. They finally got rules in place to start achieving it. The second apron is extremely punitive and limits the abilty of teams to improve themselves through the means that GMs have been exploiting for decades now.
For pure basketball reasons, there is no reason that they would have wanted to get rid of Jrue, Horford, Kornet, or maybe even Porzingis. Not a single one of those key players was meaningfully replaced with either internal promotion or external addition because there wasn't a good way to. To add insult to injury, being forced to make these moves meant Brad had little leverage to get good value in return.
You could maybe argue that they could have put off this kind of teardown until next year, but the Tatum injury made the decision obvious. There was tremendous uncertainty about the timeline of his return and how he would look when he came back. Obviously, he's back and he's been tremendous, but even his comeback may have hit a serious snag with the new calf injury.
This year was about getting out from under the aprons, and even the tax, and developing young talent to understand the what the future looks like next year and beyond. Brad did a phenomenal job of achieving that. Mazzulla did a phenomenal job of coaching this team to the second seed despite the depleted roster.
The playoffs are a different animal. The Celtics overachieved this year and overachievement has a way of getting exposed in the postseason.