The most value of Brown comes from being a very good two-way wing. That's a very scarce commodity. That's also why the Celtics should be reluctant to trade him. But just because Brown scores a lot of points doesn't make him an equal to Tatum. Obviously Tatum is the star of the team and then you have Brown, White and Brogdon who don't differ much in the value they bring to the team. Same goes for Timelord if he could stay healthy.
This Celtics team is really deep and a contender. Apart from small minor moves (like adding Muscala) the big swing to lift the Celtics to even greater heights would be adding that second superstar. And probably the only way to obtain that superstar is offering Brown.
Something that's overlooked in my opinion is that the Celtics lack a bit in absolute starpower in comparison with some of the other contenders. Tatum is great, but he's not as good as the likes of Jokic, Antetokounmpo, Doncic, Embiid or even Durant or Leonard if available. I think most would agree on this if they're honest.
But a lot of you will respond with "but Tatum has Brown" and that's a mistake, because Murray, Holiday, Irving, Harden, Booker and George are equal to or better than Brown. There's a perception that Brown is a top 20 player, while he actually isn't. Though that does speak to the incredible talent level in the NBA that's at an alltime peak.
If the NBA offered to stripe every team of his two best players I would gladly agree and pencil in a title with White, Pritchard, Smart, Brogdon, Hauser, G-Will, Muscala, Horford, Timelord and Kornet.