I fully expected this, but I think it's a mistake to bring Joe back. But before I explain that opinion, I want to first say that, in a way, I feel bad for Joe, because he was placed in a very difficult position thanks to Ime's ill-fated decision. And it wasn't Joe's fault that a couple assistants left for other teams, or that Brad failed to give him any additional assistants to replace the departures.
That said, I still think it's a bad idea to extend Joe, primarily because, as others have said, he had zero previous NBA head-coaching experience, and he had no coaching experience that was even close to being NBA-level (D2 was his highest, I believe). Also, even though Boston is still a relatively young team, it's not a team of first-, second-, or even third-year guys who are fairly new to the NBA; it's a team of young veterans who have been through several Eastern Conference Finals and one NBA Finals, so to give them a greenhorn coach who has a LOT to learn and faces a steep learning curve will, IMO, set this team back a couple years as they wait for Joe to catch up. And that's if he even has NBA head-coaching chops and does eventually catch up.
Then again, it seems like Wyc and Brad, and quite a few fans as well, don't even expect the Celtics to win a title for at least another couple of years, so I guess they're not in a hurry for Joe to catch up. But in my view, that's just wasting a couple of years of Tatum and Brown's youth. So the Cs will probably be in the ECF again next season, just because of their talent level, but that might (once again) be as far as they go.
I believe that Brad and Joe and everyone else involved wants to win titles, but I also get the impression from them that they're like, "Yeah, we want titles, but what we care most about is putting a good product on the floor and creating a pleasant atmosphere for our players and fans ... and if we happen to win any titles, that's great too."
And my response to that is: All of that stuff is well and good ... as far as it goes. I want the players and the fans to have a pleasant experience, and I obviously prefer having good teams to cheer for as opposed to bad teams, but what I want most, as a fan, is titles. Banners. To officially move back ahead of the hated Lakers in the title race (although I already consider the Celtics to be ahead of the Lakers in title count, but that's another kettle of fish).
Some people talk about process, and things taking time, and there's a lot of truth to that, but this team needs to grow beyond being a team that just gets to the ECF (and a one-off NBA Finals loss). I know, I'm just an entitled fan. I get it. But this is the Boston Celtics, for Pete's sake. Since the last Bird title ('86), the Celtics have 1 championship ... and the Lakers have 8. EIGHT. I get that L.A. is a more attractive locale than Boston to many people, but please find a way to correct this injustice.
TL;DR: Sooo ... maybe by 2028 the Jays and Joe will have the maturity and experience to finally bring home a title?