So i consider myself a member of JKJB nation, and I gotta say most of the thought process comes down to the following:
1) Jaylen brown is at this point underrated by Celtics fans. He's one of the best scorers in the NBA, literally one of the most efficient high volume guys so far this year. He rebounds well for his position. He defends well to very well especially as a switch guy. He isn't a good passer sure, maybe even below average for his position, but as a second option on a good team he's pretty close to exactly what you want. He's a similar age to your cornerstone guy in Tatum, and on a sub max contract the next three years. Teams spend years chasing that sort of second banana (Dallas for example would KILL for Jaylen Brown). I think honestly having a guy who plays the same position and is even better/ younger probably has caused some Celtics fans to under appreciate how good Jaylen is.
2) Any proposed trade has to make sense given the above facts. It makes no sense to trade Jaylen for a lateral move that's older or on a shorter contract. It also makes no sense to trade him for a "package" because we are trying to win and getting back a player + 2 firsts is not the kind of move a winning team makes. There are plenty of others things to try first (such as trading smart and getting a competent pass first PG) before you do a "shake it up" style move, get back to me in two years and maybe things will have changed.
3) If you are trying to put together a Brown trade please take into account age/contract. Lillard for example is better than Brown, Lillard is also 31-33 and making 39.3, 42.5, 46 million the next three years. Given that this team clearly has tax constraints that salary at that age is kind of a big deal. This kind of trade is also a no go.
4) I have seen no evidence that him and Tatum can't be really good together. The teams net rating with those guys on court the last three years has always been good to great. The problems have typically been when one or both is off court and that suggests a team construction issue more than a top end talent issue.
So I guess would i trade Jaylen? Ya, but it has to be for a clear upgrade, and those guys are very rare and likely just not available. So given that id use the next two years trying to make the mix around him and Tatum work. In two years when brown is entering his final year we can revisit all this.
Latsly, I just like Jaylen as a person and a ball player. I'm not trading him because we started the year 4-6. That's an overreaction. I literally put zero weight on how we've performed so far this year on the question of trading Brown. I need a larger sample size.