Why on earth would Lebron leave Miami? That roster, flawed as it is, just made 4 straight Finals, and until a blockbuster deal happens in the East are still the odds on favorite to win the East again next season. Who else is going to win the East? Indiana will most likely be worse unless they retain Lance. If the Bulls get Melo they will be a threat, but not a favorite (and you are fooling yourself if you ever expect to see Derrick Rose at full force again). The 'Zards? The Raptors? Not yet, barring a surprise trade or leap from some player they already have. If Miami runs the team back, reloading the back end of their roster with whatever ring-chasing vets they can get their hands on, they will AT LEAST make the Conference Finals, and probably the Finals.
Further, Lebron isn't stupid. He knows how much negative backlash he got from "The Decision." He cares WAY too much about his legacy/place in league history and how people think of him as a person and player to leave a team again like that. He knows it will get him labelled a mercenary and change the way people think of him both now and 10 years after he retires.
He's not going anywhere.