I have been saying something like this for basically the last 3 years, but I wanted to fully capture my thoughts as to why I think the team should trade Brown and why I believe Brown isn't all that important to the Celtics actual wins and losses.
First, some pure facts
Year - Record w/ Brown, Record w/o Brown
2022 - 9-12, 7-6
2021 - 30-28, 6-8
2020 - 38-19, 10-5
2019 - 41-33, 8-0
2018 - 47-23, 8-4
Not counting his rookie year since he didn't play much, so in total Boston is 165-115 (58.9%) with Brown and is 39-23 (62.9%) without Brown. So when Brown hasn't been available at all, Boston has won games at a higher rate than when Brown has been available.
Now contrast that with Tatum
2022 - 16-17, 0-2
2021 - 34-30, 2-6
2020 - 46-20, 2-4
2019 - 48-31, 1-2
2018 - 53-27, 2-0
So in total Boston is 197-125 (61.1%) with Tatum and is 7-14 (33.3%) without Tatum. Obviously that is a fairly small sample size, but it is a dramatic difference.
But I don't think the dramatic difference is a coincidence for several reasons. For the most part Tatum and Brown are not a very good fit and this is borne out in the simple fact that Tatum plays better when Brown isn't around. I believe a lot of that is because they essentially function best when they occupy the same parts of the floor and thus get in each other's way. So when Brown isn't available, Tatum has more freedom on the floor to play the game the way that best suits him. And this isn't just conjecture. Take this year thus far.
Here are Tatum's numbers in the games Tatum and Brown have played together: 22.6 p, 8.5 r, 4 a - 37.12 FG%, 25.67 3PT%, 78.39 FT%
Now here are Tatum's numbers in the games Brown missed that Tatum played: 29.6 p, 8.8 r, 3.4 a - 48.96 FG%, 40.4 3PT%, 80.34 FT%
This is similar for both stints Brown missed a bunch of games in a row and the games in between and surrounding them (so it isn't skewed to a slow start or a hot streak). This also matches fairly closely to how Tatum has historically played with and without Brown (at least from a shooting perspective, he quite simply shoots a lot better in the games Brown doesn't play). Tatum is better when Brown doesn't play and that leads to Boston winning games at a slightly higher percentage when Brown doesn't actually play.
Conversely, the inverse is not as true. The Celtics do not win at nearly the same rate when they have Brown and do not have Tatum. Clearly, I believe that is because Tatum is a better player, but it goes deeper than that. Brown's style of play is much more typical of what you find from a role player. What I mean by that, is he is a guy that does a lot of things well, but is not really elite at anything, and thus can't handle the load when he has to be the #1 guy. So because he can't play like a lead scorer and because he isn't a master of anything else i.e. he is a very good, but not elite defender, shooter, etc. when the focus is on him, the team doesn't play as well nor win at the same rate. Brown is what I'll call a suped up role player. He is a jack of all trades, but a master of none, so he just doesn't have the positive impact on winning, you would think someone with his stats, skills, etc. would have. And that brings us back to point #1 i.e. that because he and Tatum are not a great fit, Brown's best traits are not being properly utilized and he harms Tatum as well, thereby diminishing the ceiling of each individually and as a team collectively.
To be clear, I do not necessarily think Boston would be better over the course of a season if Brown got hurt and missed the year, however, I do think without a shadow of a doubt that Boston would be better if they got better fitting pieces around Tatum in a trade of Brown, even if the players were not individually as skilled or talented as Brown. We saw something similar to this when the team downgraded from Irving to Walker. Boston got better when they had a guy that fit better off and on the court even though that player wasn't as good. I absolutely believe the same thing would happen in a trade of Brown, if the right pieces were brought back. This is also why I would have traded Brown for Kawhi or Harden over the last several seasons even if it was a short-term thing as I do not think that Boston is going to compete for titles with the roster as currently constructed. Even if Tatum becomes a MVP type player, I just don't think the pieces are in place to properly support him. That is why Brown must be traded. It has to be the right trade, but I do believe it must happen at some point soon (i.e. this year or this summer). The longer the team goes like this, the more likely it is that Tatum leaves and that would set the franchise back years and we would have wasted a ton of years and essentially wasted the 2nd best player the franchise has had since Bird (KG was better for 1.5 years until he got hurt). And that would be the real shame. To have a talent like Tatum and waste it by being afraid to pull the trigger. Ainge failed on that for years, hopefully Stevens does not.