I see this as a clear downgrade.
1. Simons two biggest strengths as a player are his scoring (career 19 points per 36) and three point shooting (career 39% 3PT). Brogdon who also has career averages of 18.9 points per 36 and 39% 3PT which is dead even with Simons - meaning the biggest strength Simons offers still gives us zero benefit over Brogdon.
2. Simons is a terrible defensive player and is also a pretty terrible passer/playmaker by PG standards, averaging 3.7 Assists Per 36 and 1.85 AST/TO. Brogdon is a significantly better defender (he's at least passable) and is a massively better playmaker (5.1 Assists Per 36, 2.6 AST/TO).
3. Brogdon has demonstrated that he's OK with coming off the bench if that's what it takes to help the team win, and he's clearly demonstrated that he's capable of producing at a high level in that role - he won 6MOTY this past season and was instrumental to Boston having the league's best record for most of the season. We have no idea if Simons would be happy to come off the bench or how effective he would be in that role.
4. Health for Brogdon is a definite concern, as he's played only 69% of possibly games over his 7 year career and has played over 65 gamers only twice in his 7 year career. Sadly though Anfernee Simons doesn't solve this problem for us. He's played only 66% of possible games over his 5 year career and has played over 65 games only one time - he's really no less a risk then Brogdon health wise.
5. Brogdon has 6x more NBA experience (1,200 vs 200 minutes) - something that is absolutely crucial for the Celtics now that they are firmly in "Win now" mode.
Honestly. I'm struggling to see any tangible advantage in having Simons over Brogdon outside of the fact that Simons is younger only 24 vs 30). And given the Celtics current aspiration, Brogdon's age (and the experience that comes with it) may actually be more valuable then Simon's youth.
Personally, I don't have much interest in moving Brogdon at all unless it's part of a deal that brings us back a seriously game-changing piece. For example if we come somehow move Brown and Brogdon in return for Giannis and Jrue Holiday (highly unlikely) then I'd do that in a heartbeat.
Beyond that I really don't have a lot of interest in moving Brogdon. Yes, there is some risk around him health-wise. But he did play a pretty respectable 67 games last season (at a solid 26 MPG) and in the games he played his impact was felt in a huge way. Brogdon and Tatum were the only two Celtics players who ranked top 4 on the team in BPM, WS/48 and VORP.