This is all correct. You could trade JB for someone whose game is more complementary to JT's, sure, but you would be downgrading in talent.
JB isn't a ball stopper. He's a fine fit for Tatum, who isn't going to go for 50 every night. Jaylen can take over possessions, but also be a great shooter when Tatum kicks. He's also a menace in transition. It's a stupid play to trade him unless a top 10 player shoots his way out of town, and maybe even then.
If he were a little less robotic in his playmaking, Jaylen would be a great fit with Tatum. As is, he's a good one. I think Jaylen Brown is one of the most underrated
shooters in the league. He's also not horrible defensively, although this year it looks like he slipped a lot. The C's did as a whole though.
A team needs at least 2 scorers (guys who can get their own shots) anyway. I think Jaylen and Jayson would work even better if they had the right players around them. I think the non complementary parts are Kemba and to a lesser extent, Smart. Jaylen and Jayson are not the problem, they are part of the solution.
Everyone else is the "problem" for various reasons ranging from health to fit to inexperience. TL is a great fit if healthy. AN is a great fit if he turns into that 16-20 point scorer on great shooting, decent rebounding and passable defense. Langford is a great fit if he's healthy and develops any type of offensive game. TL is a problem because he's never healthy. AN is a problem because he's unproven/inexperienced. RL is a problem because of injuries and being unproven/inexperienced. I could go on for every other play on the team.
Personally, I keep JT and JB. I try to hold on to AN. I try to extend TL really cheap for 2-4 years, barring that I explore his trade value. I look into an S&T on Fournier but don't hesitate to bring him back if there aren't any good offers. I try like hell to move Kemba and would attach anyone not named JT, JB and AN to do so, including the 2021 pick. I know some people would disagree but I'd gladly give a team Romeo or the pick if it meant moving on from Kemba. I'd gladly give both if there were other deals lined up. Id consider AN too if it meant getting a player the caliber of JT or JB.
Depending on what happens with Kemba, I'd either keep Smart to be the PG or look to trade him for a more traditional 3+D guy if the C's acquired someone like Lonzo to run point. If they use their pick, I'd pick BPA and hope it was a long athletic big.
I don't think anyone else really has enough value on their own to be traded. I'd might try to move on from TT too for a cheaper option and if that didn't work out, pick up a big at the deadline or after buyouts because they seem to be readily available.
Depending on the poster, the C's probably have anywhere from 2 to 6 keepers (JT, JB, TL, AN, EF, MS). I'm sure Kemba and TT have some support but I doubt there would be much uproar if they were traded away. I have 2 hard keepers (JT, JB) and 2 soft keepers. (TL, AN). My 5th keep would be the 1st round pick.