If it were anyone but the Heat (or the Lakers) I might be slightly more tempted by their offer. But the Heat just are never really bad for very long. Stars want to go there, and they're incredibly well-managed. Just look at their record--since 2003 they've had one season where they dipped below 37 wins.
I mean if this were the Bulls or Wizards, then yeah, maybe.... Or, more realistically, fringe contender (or think they can be contender) teams like Minnesota or Boston or Philly. Someone like that is bound to get frisky in a few months (assuming Dame avoids injury) and we get at least a similar package from a team who isn't run as competently (and therefore is likely to yield better picks several years out).
It's all a gamble. But recent history has shown that the team that is patient in trading their all star gets a better deal.