They 100% are not trading JB for KAT. He is already inked to a Super Max contract, so he has all the same long term cap issues that just signing JB would have. Then there's the fact that he is a player who has to play center, the most important defensive position, and is bad at it. That's a hard no.
But this deal is even more unrealistic. Sure, you can get to the salary. And maybe even MIN would consider it. But Boston would never do it. Three super max guys going forward doesn't work financially. Its a no go.
Towns contract is actually a little different than what Brown's will be if he extends. Towns will make $36M next season, Brown about $32. After that though, Towns will have 4 years and $225 remaining (starting at $50M) where Brown would have 5 years and $295M remaining, at least that seems to be the number being reported. I think to get to $295M, Brown's contract would have to start a little higher. Not all that much difference though.
I agree that there are major hurdles to this deal happening now. Brown could be a nice piece for MIN, but they would be taking the chance that they would not be able to extend him. They have Towns all locked up right now. I suspect that MIN is going to run it back with Towns and maybe trade him next off season, when his contact balloons, if they trade him at all. From the Celtics perspective, I see Towns as comparable talent to Brown, all star, occasional all NBA. But he plays a position that is more complementary to Tatum.
I feel on the Celtics, that Towns would be a perennial All-NBA caliber player. He is not quite Jokic or Giannis or Embiid, but one of the top bigs in the game. I don't care if you want to call him a PF or call him a C. That is how teams win, a top big and a top wing. Right now, we have 2 top wings but not close to a top big. I admit to having some hesitation but if MIN were interested, I give this a ton of thought, and probably do it. There is risk in making such a big change to what is already a good team, so as I said, I have some hesitation, but I love the thought of Tatum and Towns, the 2 Ts instead of the 2 Js.