I think the biggest reason why a Jaylen Brown supermax is so hard to fathom for some of us in Boston is the fact that we have Tatum, who kinda overshadows Jaylen Brown.
Looking at Brown in isolation in any other scenario / on any other team, if you have a 25/26 year old fairly healthy wing whose averaging upwards of 26/7/3, improving every year, and just made an All-NBA team then offering that player a supermax would be pretty much a no-brainer. Any and every team would do it without a second thought regardless of whether that team is a contender or a young developing team, a player like that wold be considered pure gold.
The thing that makes it seem so crazy for us is that we just happen to be lucky enough to have another 24/25 year old very healthy two-way wing averaging upwards of 30/8/4 who is improving every year and just made an All-NBA first team and is generally considered a top 5-10 player in the league.
We all know that Tatum is getting a supermax as soon as the opportunity presents itself, and the idea of have both two young wings with as-yet undetermined ceilings both signed to long term supermax deals is a little scary, because *IF* these two guys don't prove good enough to lead the Celtics to the promised land, then everybody knows that our ability to add more talent around those guys i going to be very limited.
So I think the hesitation for some to give Brown a supermax is not so much a reflection of who Brown is as a player, but moreso a reflection of the fairly unusual situation Boston currently finds themselves in. Especially given that Tatum and Brown are such similar players overall, that there is some feeling that we are committing the entire future of the team to Tatum and a second slightly less good (but who seems to think he's just as good) version of Tatum. And many of us still have question marks as to whether these two guys fit together well enough (as both players and in terms of prsonality/cultural fit) to form the core of a championship squad.
For me, personally, I did have major concerns about this...but I have to admit that I feel much better about it now that we have Porzingis locked in for the next 3 years. I know Porzingis is a health risk, but I have felt for years that the Celtics desperately need an consistent offensive presence in the front court to help take pressure off the Jays and bring more balance to the offence, and having that third offensive weapon to take pressure off the Jays makes me feel much more comfortable committing to both Tatum and Brown long term. Tatum, Brown, Timelord and Porzingis all have 5-6 years of prime basketball left in them, which means the championship window for this team would be huge - there is a high probability of multiple NBA finals runs, and a pretty high chance of winning at least one title. If all goes really well there is potential for a genuine dynasty.
We'll see how it goes, but I like where the team is and I've grown pretty comfortable with the Brown supermax.