I don't see a feasible way to implement a minimum salary that wouldn't inevitably screw over teams that'd have to overpay guys far what they're currently worth due to past achievements. I mean, using your Miami example, it'd be pretty ridiculous to force Miami to have to offer D-Wade at least $17 million given his decreased abilities. Sounds good and everything if you hate the Heat, anyway, but what about when that affects your own team?