Sunk costs is about all I can say about this trade. Yes, it's a heartbreaking return, but if someone was offering more we would have taken it.
We'll see what happens, there's a lot of rationalization involved here but a few things DO seem to be true:
1) Analytics don't love Brown, much less a $70M / year extension. I do believe this can be taken too far, but the fact remains that he's a below-average 3PT shooter, below-average FT shooter, poor passer and his +/- is probably not an aberration after many years of trailing other top players. He's more productive than ever, but this league MVP stuff is ludicrous except for Tatum's injury.
2) Brown considers himself an MVP-level player and has developed at least a bit of a smug, smarter-than-you attitude that can go south fast in a locker room. He's no Kyrie, some teammates may love him, but he's almost certainly rubbing SOME people the wrong way. Risk of butting heads in the next 4 years (with extension) is through the roof.
3) Tatum remains the better player, and it's not real close. Some people prefer Brown's *style*, and an any given night or series there's a chance Brown plays better. But every measure at any scale says Tatum is the guy.
4) Boston is an analytics house. There is no other measure - that IS reality to them. In their world, White is likely the better player and George is just fine as a stop-gap. He's more efficient than Brown and White / Pritchard / others can pick up the usage making a better overall offense. In theory. They don't see a steep decline.
It is what it is... we should still be favored to win ~55 games this year. What we need is Tatum healthy, top defense, and locker room chemistry. If New York can run off a title, we remain at least a fringe contender.