Setting aside whether you think Brown is a guard or a forward, if you were building a team, who would you rather have as your shooting guard:
Lillard
Mitchell
Brown
Maxey
Lavine
(just trying to capture all the top East Conference non-PG guards).
The fact that Brown has the size and defensive ability to play at SF also, does not take away from how effective he is or can be as a SG. It actually just adds to his overall value to a team.
Up until last season, the Celtics have played with 2 bigs, Tatum at SF, and Brown at SG, throughout Brown's career. It is true that he is being played as a forward this season (if you adhere to the 2 guards, 2 forwards, 1 center convention vs. just saying that we are playing 3 guards), but he has played more as a guard in his career and in my opinion, is at his more natural position when playing guard.