Agree with the above post. I think trying to point to one thing -- effort, play style, shooting (as I have), health, COVID, miscommunication due to limited play together, coaching struggles -- over any other one factor is the wrong thing to do.
All of these things have played a role, and sometimes it's not clear in an individual instance exactly what's wrong. There were times this season when I couldn't tell, for example, if Jaylen wasn't playing hard on defense or if he was just trying to avoid hurting his knee more. Take Fournier: I don't know if it's COVID recovery that's bothering him, or if he's just spent so many years on lackluster defenses that all the switching has him confused, because he's blowing assignments all over the place.
I'm still in the camp that thinks this team is better next year even just because of improvements from a few players: I expect another leap from Tatum, maybe another leap from Jaylen (at least in the volume of 3's he's taking, please God), and for Nesmith and Pritchard to contribute a lot more.
But a little bit better may not be enough. Ainge has two major dilemmas, and I don't think either of them are CBS: Kemba and Robert Williams. I think he's the kind of player you dream on, but once he's injured, you're stuck playing TT 30 minutes a game, and that's just not sustainable. TT has never been a rim protector, hasn't been durable for some time, so at this point I'm not sure I'd bet on his health. As others have pointed out, it's difficult enough to find excellent, durable players at that position without fielding two of them with these limitations.
Options? Jarrett Allen is up for his RFA. I expect Cleveland will try to pay him, but who knows. He would be the surest bet as a contributor of the players I've been thinkin about. Then there's Utah: Utah paid Derrick Favors quite a bit of money, but he's only getting 15 minutes a game over there, and he's still capable of challenging guys at the rim. He's got his own history of injury issues, of course, so I'm not sure I'd take him on except as a flier. Kelly Olynyk is a free agent, too: he's no good as a rim protector, but he offers a lot more on offense than TT does, and has been putting up some crazy numbers in Houston. There's Nerlens Noel, too. He might be another guy it'd be nice to give some run. Khem Birch is also an interesting player out of Toronto, and will be a free agent next year. It's also possible the Celtics stick with the Kornet--TT--Williams trio and end up being terrible defensively (again).
I hope the Celtics find a market for Kemba. I hope his resurgence is enough to convince some team desperate for offense to take him on. Because I honestly do not expect it to continue. Wrong side of 30, knee injury, small guard. The cliff is right there, and all it will take is a little push.
Smart's another problem. I think the team can use him, but not if Stevens starts him, and not if he regularly throws up 10-15 shots a game when there's plenty of other better options on the floor. He needs to be in the corner on offense and play 25 mpg. For all his playmaking ability and his ability to protect the ball, he continues to be an abysmal shooter. The only thing -- the only thing -- I find encouraging about his offense is that he appears to have gotten better around the rim the last three years. If he would just stop chucking up bad 3's, his percentages would head towards acceptable territory. But then, we're on year seven of him foisting up terrible shots, so I don't necessarily see that changing.