In my humble opinion, Jaylen Brown has peaked, and I don't see him take another step from here. This is his peak athleticism and shooting. I don't see him make another leap with his ball handling, passing or defense from here. He has had the good fortune of being generally healthy and making steady improvements throughout his career thus far with his work ethic and I commend him for it.
So lets say going forward, he keeps the pace of 24-27 PPG, 5-7 RPG, 3-4 APG, 2-3 TOV - are you satisfied to pay him 50 million/year all the way up to 65 million/year the last year of his contract?
When Tatum's contract is due, there is going to be tough discussions taking place. There is no way to maintain the current roster no matter the financial gymnastics and a lot of culling will need to be done. Either keep the duo and gut the rest of the team or trade JB in favor of the rest of the team.
The no brainer decision in this off season was to give JB the contract he demanded while obtaining Jrue and KP - this gives us the best shot at a championship this year, while maintaining some roster stability, after Smart/RWII/Brogdon was traded. However, I think the likely decision in the future will be to trade JB in return for a high value younger player(s) and picks.
JB is a steady and consistent offensive player (above average, hence the contract he received), however, he is not a cornerstone player of a franchise IMO to demand such a contract. He not an above average defender, he is not a great passer or playmaker, and his court vision is limited. I think you can't handle this level of a market cap hit from a player with these limitations.
In the past posts dating years back, I said Rondo, Bradley, Kemba, Marcus, etc would all needed to be traded for the Celtics to take the next step and I was ridiculed for it. IMO, this will be the same reaction I expect, but to a greater degree, but let this post sit in the internet and let's revisit in the next years to see if I am proven right again.