Oklahoma City missed the playoffs for 3 seasons and one of those 3, they won 40 games. They weren't bad for long at all and are more similar to Boston in that they had aging stars with value. The trading of George and Westbrook is what kick started the OKC rebuild. And Boston has the 3rd player such that if they traded White and Brown, they'd still have Tatum. OKC is more unique than most situations, but because of the starting point, this Boston team (much like the KG/PP trade) isn't starting from the bottom. The tear down itself is position of strength to jump start a rebuild.
I don't see winning 40 games as the highlight of a 3-year playoff drought as being something BOS fans would be too happy with. Yes, we would have Tatum for that stretch but OKC had SGA. SGA + spare parts and good prospects was a bad team for 3+ years. Now they are a good team.
And to make sure it is clear what I am debating, it is not to trade or not to trade Brown (or White), it is if you do trade either in the near term, what do you trade them for. If you trade them for spare parts and draft picks, the team will be worse, not better. Maybe the team is better some time in the future, maybe (no guarantee), but by then Tatum will be in his 30s. The time is now to do what you are going to do with Tatum.
I am OK to trade Brown or White for good players who help win now. It may get to a point sometime in the future where BOS needs to suffer through a 3-5 year tank to totally rebuild, which would include trading Tatum at that point, but I am for delaying that as much as possible.
In summary:
Trade Brown for Giannis (for example): Floor is making the playoffs every season. Ceiling is winning a title.
Trade Brown for Spare Parts and Draft picks: Floor is no playoffs for 3+ seasons. Ceiling is to have a title contending team in 3-5 years maybe.