If you look at the +/- numbers, Tatum is better with Brown this season than without.
+/- numbers can be weird. White always has had great +/- numbers his whole time on the Celtics. But I don't see White as a better player than Brown.
The thing with Brown is that he gives us a great option when Tatum is out or just not having a great game. Now that we have added Porzingis, I think that is going to allow Tatum and Brown to be more complementary. I don't see that the problem was ever with Brown and Tatum, the problem was we didn't have that skilled big to take the pressure off either of them.
I can see that in the ideal world, the team you would build around Tatum would be #1, a skilled big (we have that now). #2, a skilled ball handler or floor general type (not necessarily a traditional PG). That would be Holiday as an upgrade over Smart. #3, you would want a secondary high end scorer/wing, that is Brown's role. He can score and defend multiple positions.
Once Holiday and Porzingis settle into their roles, I think you are to see Brown and Tatum work just fine together. If you were to trade Brown, what alternate need would you address? Would the team be better with another all star level big, more of a defensive blue collar type (Gobert)? Another all star level PG/ballhandler? I think the secondary all star level scorer/wing is more valuable than any other option.