Butler is a terrific player, but he is big money and plays a position they are already pretty strong in. So why waste resources on a strength?
No matter which guard you trade for Butler, you're going to have chemistry problems. Rose-Butler competed for shots. IT-Butler will compete for shots.
I don't know if I would trade Bradley. He has a great contract, is an elite player defensively, and seems to improve offensively every year. The one negative I have against him is he's been injured twice in the playoffs, which has cost them. They had the Heat beat in the finals a few years back when his shoulder gave out. And then this year, his leg injury killed them against Atlanta. He and Smart are their defensive backbone, along with Crowder. Keep them intact.
I'd be happier if they put their money in an offensive stud up front. And maybe another big who can rebound and block shots.