You can't eat legacy. 60% of all NBA players go broke within 5 years of retirement. The money won't be there forever and in the end it's far more important than some imaginary thing called legacy. Besides, it's their life, they have to live it.
Wilt Chamberlain played for 3 different franchises at a time when players had no control over where they played. Despite being the most dominant player ever he only finished with two titles, both of them later in his career. I'm pretty sure he had all the legacy he could handle.
If this "legacy" thing is mostly about Carmelo and Bosh, keep in mind the choice is between getting maximum salary versus having a chance to win it all. Winning it all is huge but we're just talking about the chance to win it all. It's hard to give up the sure thing of money for the not-so-sure thing of winning it all.
Steve