Nothing. What leverage did they have? If he wanted to go to Miami to team with Bosh and Wade, they would have been beyond idiotic to trade for LeBron last season. Would any team in the league reject any sign-and-trade for the best player in the game? I can't see it. It'd leave them in ruins with every player for decades. It's not like LeBron was going to stick around Cleveland if it fell through.
Houston and Dallas came out saying they'd be willing to trade for Melo even if he wasn't going to re-sign with them, I'm sure teams would have done the same for LeBron. The positive in trading him last season would've been the Cavs could send him wherever they want, it'd be upto the other team to decide if they want to take the risk in essentially renting him for half a season. I bet the Knicks would've even gone for it with a similar package to what they offered for Melo just to get a head start on NJ/Miami and the Bulls when it comes to signing him.
I could pretty much guarantee you that if every NBA player of the past played under the current CBA and league they'd have done the same shit. WILT-BAYLOR-WEST? Wilt on the Sixers didn't just happen either.
I'd doubt it just because it's not like it happens often under this current CBA. For every spaz like Melo/LeBron (not Bosh, he's not a star) there's guys like Kobe, Durant, Dirk, Duncan, Steve Nash even (he was forced out of Dallas obv, would have stayed otherwise) who stick with their teams and want to win with them as the focal point. You think LeBron in his prime would have stayed with Cleveland if they were winning 35 games like the Lakers did after Shaq was traded? And the Deron Williams situation shows that some owners still have balls and aren't letting the inmates run the asylum. Deron didn't want to be in New Jersey, but too bad he's stuck there for one more season after this one. There's a chance he won't re-sign there but the Nets took that risk Utah made the move because they didn't want to be handcuffed next year and having to decide between trading him to whichever teams he hand picks.
LeBron's a life long bandwagoner (Yankees fan, Cowboys fan, Ohio State fan), I hope he never wins a championship