Tatum is better than SGA was those first few seasons in OKC when the team was growing. Boston won't bottom out. They could have this year, but won't going forward. They need to get younger players and draft picks from other teams and hope they end up high picks like with what happened in the KG/PP trade.
I think you are proving my point. SGA is one of the best players of this generation. Dybantsa will probably not be as good as SGA. Maybe he will be, and maybe he will be no better than Markelle Fultz. But even SGA took about 4 or so seasons to be as good as Brown is right now.
So for the next 3-4 seasons, critical seasons for Tatum's window, what is the likelihood that Dybantsa is going to be better than Brown over these critical seasons? By the 4th or 5th season, sure, maybe Dybantsa passes Brown, maybe. And maybe by season 6 or 7, Dybantsa is the MVP, like SGA. It is also a real possibility that he is never as good as Brown.
This is why to me, it makes no sense to trade Brown for picks or prospects, while keeping Tatum. It would be better to trade both Tatum and Brown in that case. But if the objective is to build the best team possible to win with Tatum, for say the next 2-3 seasons anyway, then it only makes sense to trade Brown for other good players. Or of course, just keep Jaylen Brown.