Jaylen Brown is basically a "fringy" All-NBA player. He's clearly a Top 30 player in this league, and fits in with most that are in the Top 15-30 range, where in any given season any player in that group has a shot at earning All-NBA.
Here's the thing: he's 26. There are a number of players in that Top 30 group that are clearly on the downside of their careers and are unlikely to be Top 30 in a couple of years: Harden, Kawhi, Paul George, just to name a few. So it becomes extraordinarily difficult for the Celtics to upgrade or even make a lateral move if they were to trade Brown before the supermax. Older players in this range are always an injury/regression risk, which would cause this team's window to slam shut quickly. Teams are unlikely to move on from the younger players in this group (aka, Mitchell, Bam, etc.) or those that have potential to join that group (Haliburton, etc.).
Now, if you don't believe Brown is a Top 30 or even Top 50 player in this league, then I understand the angst. I have difficulty accepting arguments that Brown is not Top 50. And it's a stretch to me that he's not Top 30 although I get why the on/off related stats give people pause. And I think a Top 30-50 player is still tradeable under the supermax given the prime years of control and the expected cap increases in the future.