I understand what you're suggesting, and you may not be wrong, but on the other hand, nobody the Celts have right now is on Butler's level, and there's nothing like a guarantee that they'll get anybody in the next 3-4 years that approaches his value.
I think Butler is at a bit of a disadvantage in a team USA type setting because his shooting is less than phenomenal and his on-ball skills, while excellent, are not quite at the same level as, say, KD or Irving. He's just not going to "pop" when he's playing mostly off-ball in a lesser offensive role.
Butler's value really shows through when he's utilized as "the guy" on both ends. Which is another way of saying that his value lies in the fact that he can carry that kind of heavy load (ie he's exactly the kind of wing the Celts are missing). If he's the 4th option out on the floor and doesn't handle the ball much, yeah, he's kind of an extreme luxury.
Would swapping Jaylen Brown and Avery Bradley -- for example -- for Jimmy Butler make the Celts a contender?
Probably not, but they'd be the clear-cut 2nd best team in the East, with a core trio that forms the holy trinity of All-Star talent at guard, wing, and big. Not a bad spot to be in.
I tend to think the Bulls were asking for all of the BRK picks plus Bradley or Crowder, maybe more. Not worth it.