I agree that Brown is not insanely overpaid. Maybe a little but in a couple of years, it won't seem like it so much. He has been 3rd team All-NBA which puts him in the top 15, at least in that regard. Top 20 for sure. He could be a decent #1 on several teams.
As to trading him, I don't think this knee thing is that big a deal unless he has the reconstruction surgery which comes with 6-9 month recovery. I don't think it is likely he is traded in any case, nor do I think it is entirely off the table. I think if DAL was interested in trading Flagg, that BOS would return that interest. DAL needs to start doing some winning to justify the Luka/Davis trade. Brown could go a long way towards that. Flagg, not so much in the near term. DAL would have to send back a lot though, unless they finagle something with a 3rd team.