Yeah the whole "I want to win with the Jays" thing is fine if you're also willing to say "Winning a title isn't that important to me", because to an extent that is what you are saying, that you'd rather have Jaylen Brown on the team even if it means not winning a title. And to me... yeah I don't feel that way at all. We drafted Jaylen, that's awesome, he became better than we reasonably hoped, also awesome.... he's about to get paid, and to me, he's probably not going to be worth the money, add in off-court questions... and yeah, give me the at times surly guy who is a top 15 all time player who gives me a better shot at the title. I'd much rather win with Tatum and Durant than lose with Jaylen and Tatum.
Edit- and that ignores that your window to win with the Jays might be this year only when they don't give Jaylen 5/290 and he demands a trade, or 2 years when they let him go to FA and he chooses to have his "own" team rather than playing Great Value Pippen on the Celtics.
If Brad doesn't think Jaylen is going to re-sign, because he wants his own team or something like that, then of course you have to pursue it. But otherwise, there's no question that they should sign him at the max, as opposed to just not paying him. I think he's absolutely worth 5/290, especially to the Celtics. Jaylen's a top-25 player who is still getting better. He's limited in some ways, but he has shown the ability to successfully play off Jayson Tatum. With him, they're a title contender, without him, they're not. I mean, I think signing Jaylen to the max is an absolute no-brainer. What are we talking about here? Those Supermax contracts are bad business when you're overpaying a marginal star to be your number one: Signing 29-year-old Bradley Beal to a Supermax to be your best player is Exhibit A. Most teams don't have the luxury of having a second option that's good enough to qualify for the Supermax, but if they did, they'd pay him. This is a good problem to have. It's not like the cap space can be spent elsewhere.
Point being: it's reasonable to think that maybe the Celtics should explore a trade of Jaylen Brown for another star that better complements Jayson Tatum, whether it's Durant, or Donavan Mitchell, or someone else (unless that someone else is Karl-Anthony Towns!). I don't think that letting Jaylen walk for nothing, or trading him for something less than another top-25 player just because you don't want to pay him, is a reasonable path forward.
And I reject the idea that preferring to keep Jaylen rather than trade for Durant is tantamount to saying that winning a title isn't that important. This team was in the Finals 8 months ago, with a significantly worse roster overall, and a lesser version of their two best players.
I actually think, if it's a reasonable bet that Jaylen re-signs, that the chances of a single title are roughly equal between having Jaylen around for the next six years versus Durant for the next couple. Accounting for durability, performance decline, etc. Injuries are impossible to predict, but with Durant, I think it's likely he ends up like late career Chris Paul - brilliant when he's there, but at some point, betting on him to be healthy for four playoff rounds is a bad idea.