You cant have it both ways. Either they want to dump him and dont want to rebuild around a 26 year old 'superstar', or they don't, in which case you're going to have to overpay for someone who does the same things as a guy you already have locked up with a steal of a contract. So you get rid of one of the pieces or performs and dont replace any of the pieces who dont.
Except they're not the same type of player at all, so your entire argument is off basis to begin with.
Jimmy Butler is a 6'7" Small Forward who guards players like LeBron and Carmelo, and does so with great success.
Avery Bradley is a 6'2" Shooting Guard who guards players like Chris Paul and Kyrie Irving, and does so with great success.
So okay, for the sake of argument, let's say they are similar players, even though they play two entirely different positions on the court.
The Celtics have a gigantic glut of young Shooting Guards waiting in the wings, such as Marcus Smart, Terry Rozier, RJ Hunter and James Young.
At Small Forward, the Celtics have the solid and hustle friendly, but mediocre, Jae Crowder and..............nobody else.
So now take a step back and review the situation: trading Bradley in a package for Butler makes a ton of sense considering you have a ton of young SG replacements, especially Marcus Smart, who after another year of seasoning, will essentially BE Bradley, and maybe better. At SF, you have...no one.
You trade Bradley for Butler every single day of the week and you don't think about it for a split second. If you don't see the big picture you have little knowledge of team construction and depth in the NBA.