The legacies of both LeBron James and Tim Duncan are going to rise whether they win or lose. Tim Duncan doesn't have much more to prove in his hall of fame career and LeBron is still young enough to continue to build upon his legacy whether it be with Miami or not. There is no guarantee that if Tim Duncan were to win these Finals he'd make loud mouth Mark Cuban happy by retiring. In fact, I don't think Duncan would retire even if he won because he's not declining, he may be playing less minutes, but he's not declining. Duncan's player efficiency rating was in the top 20 in the NBA this past season. Top 20 is impressive for a player Tim Duncan's age since most players his age would be lining up their next career as basketball announcers or analysts.
In my lifetime, I've seen a team three-peat twice and I've seen several other teams dominate the league and capture multiple championships. There is no parity in this league. There are just a handful of teams that have won the championship in the last ten years. So watching one team that has dominated recently and one that has had spurts of dominance since the mid-2000's isn't really interesting to me. Do I care if the Heat become a dynasty? Not really. Do I care if the old guys walk away with their 5th title? No. When a true underdog makes it to the Finals that's when I'll feel compelled to watch.