The Celtics are in a pickle being a second apron team facing a MASSIVE payroll bc of the luxury tax. Trading Jaylen Brown isn't ideal. Jrue or Porzingis are the first options. But there are significant obstacles to moving them. This trade gets them closer to ducking the second apron, but they'd still need to make another move, probably sending out Hauser. But they get back two solid, young wings in Vassell and KJ, the #2 pick, ie, Dylan Harper who's basically Cade Cunningham 2.0. Another FRP and 2.5 SRPs. And in theory, even if Boston is bad enough, they own their own FRP in 2026, which could be top 10.
San Antonio doesn't get Giannis or KD here (who would also require more picks). But they do get a 28 year old elite wing and champion in Jaylen Brown, who would fit perfectly next to Fox, Castle, Barnes and Wemby. With the free agent signings, this immediately puts San Antonio in the top 2 conversation with OKC for the next half decade. Cashing in a rookie of Dylan Harper for a win-now move with Brown isn't something everyone will agree with. But San Antonio still has plenty of picks and assets for future trades, and there's no guarantee that a 7'5" big man will be available for 65+ games every year (Google the history of guys who were over 7' tall, and their careers being cut short bc of injury).