I think you've got it backwards a bit here.
Yes, Brad is a really good coach, which means he's good at getting a team lacking in talent to overperform.
But you give him a roster that is replete with talent, and yet we're not seeing the team exceed expectations. Why?
I think there are diminishing returns on what a coach can do to make a team play better than the talent on the roster.
A great coach can take a bunch of role players and figure out how to win games they maybe ought to lose.
But if you give a great coach great talent the ceiling of the team is probably still about the same, which is to say high, but very dependent on that elite talent fitting together.
So I think it's wrong to say Brad *needs* an underachieving team. Rather, I think it's fair to say that a great coach isn't necessarily going to make the same kind of difference with a roster flush with talent. The expectations are already sky high and there's nowhere to go but down.
That's why I think your instinct to replace a bunch of the current supporting cast with role players who are maybe less talented but whose skills and roles are more defined is on the money.
The Celts have plenty of talent, it's true, but talent doesn't win games on its own. You need to be able to have that talent all pulling in the same direction. The more talented players you have on a team, the more diminishing returns you're gonna have when their skills don't necessarily complement, but instead create confusion as to who is best suited to do what.
I think the Celts would be doing better if they just had Kyrie, Tatum, Horford healthy and good to go, Smart and Hayward as glue guys / defenders, and then a bunch of veterans with well defined skills.
Of course, that's not the team. Well that it isn't, I suppose, because it means the Celts have better depth to withstand injuries and they have assets to make moves.
But right now I think it's fair to say that the team's roster assets are not all optimized for winning right now. They're simultaneously trying to develop guys like Tatum, Brown, Rozier, etc, rehab Hayward, and also build rapport among the core group while fleshing out the supporting cast.
That's a lot to have going on in the midst of what is supposedly a contending season.
Most contending teams have a much clearer sense of who they are and how they're going to win tough games.
I agree with your theory about diminishing returns with a great team, but an issue is Stevens has his high 3 point shooting / low FT, modern bigs / lack of emphasis on rim protection system, and it seems like it's best suited for less talented players.
When you have studs, or even in general, you need to be tailoring the system to the players you have. That's why Pop has changed his offense so many times. It used to be thru Duncan in the post, then with Parker's drives and movement, then Kawhi and a ball movement system and later a Kawhi-focused system (both with lots of 3 point shooting), and now they changed it again with LMA and DeRozan.
I haven't seen Stevens change what he's doing once really. The better players you have, the less you should just be chucking up 3's. They can do more. Winning on 3's is how Butler gets upsets in March Madness vs the Dukes of the world. But the thing is, Stevens isn't coaching Butler anymore. He's coaching Duke, and I'm not sure he's figured that out yet. I hope he does soon.
This reflects my own thinking.
Stevens has a system, and everyone has to fit it. He doesn't tailor his system to the talents of the players. Or perhaps to the values of Danny Ainge. He gets a lot of leeway. At least so far.
The shocking loss to the Cavs in the 7th game last year may have alarmed a few people. That was unexpected. All those 3P robots out there shooting 3s.
This year, the poor start, with little thought, it seemed to the chemistry of the starting 5. Playing the money.
Stevens has said he never could recruit top bigs at Butler, so he tailored his game to small ball. That doesn't always work in the NBA.
The Celtics are weak in the paint, and perhaps it reflects the Stevens' system. The closer to the basket, the easier the shot. That seems to have been lost in the 3P game.
Stevens may have to alter his ways some if he is to succeed in the NBA, and bring the Celtics to another Championship.