Boston is FAR more likely to wina title in the next 4 years with Durant than it is over the next decade with Brown. It really is that simple.
Given current roster construction, how many other teams do you consider better than Boston today?
I think Milwaukee is the best team in the East, with the C's as the clear #2. The west I think 3 or 4 teams that are at least on par with the C's (Clippers, Warriors, Nuggets and maybe Grizzlies). With Durant and maybe Curry for Brown, Smart, and Grant, the C's are pretty clearly the best team in the sport. White, Brogdon, Durant, Tatum, Rob, Al, PP, Curry, Gallo is basically unstoppable. Part of the reason, GS could shut down Tatum is Boston had no one else that can create a shot or take pressure off. Brown just isn't good enough, Durant is.
I said this right when the season ended and it holds true today, Boston needed to look at this offseason like Middleton didn't get hurt and the C's lost in the 2nd round to the Bucks or else they will end up like the Suns. A 2nd round loser doesn't think they are all that close and gets a guy like Durant when he is available.