LeBron only took 1 really bad team to the finals. In 2007 he took a bad team in a historically bad eastern conference to the finals where they got swept. Then the Celtics put together their big 3 and LeBron missed the finals for 3 straight seasons, in 2 of which he didn't even make the conference finals. So realizing that he does in fact need a great supporting cast, he went to Miami where he, Wade and Bosh all took less than max money in order to play together and get pieces like Mike Miller (and subsequently guys like Battier and Ray Allen). Those Miami teams were loaded. You dissed Bosh a bit earlier, but he's a borderline hall of famer and the perfect PF to pair with LeBron since LeBron can't shoot and needs to be paired with big guys that can score outside of the paint.
That first year in Miami, LeBron wasn't even the best player on his team, Wade was. It was never more clear than in the finals that year when LeBron cowered under the pressure only to be ripped into by Wade for playing like sh1t. Miami's roster was way more talented than Dallas's that year. Guys like Magic, Bird and Jordan never got dressed down by a teammate like that in a huge game.
So he finally wins a couple of rings, Wade starts to look like maybe he has too many miles on him and they lose in the finals in 2014 to a team whose strategy is simply to make LeBron actually shoot the basketball. What does LeBron do? He dumps those guys to go back to a team that had received 3 consecutive #1 picks. He basically forces them to trade Wiggins for Love because he want's a roster that is stacked and ready to win immediately. Sure, Love got injured and missed almost all of the playoffs, but Kyrie is a great player and it's really hard to say that Cavs roster last year was less talented than the Hawks team they beat in the conference finals.
You point to the Cavs being about 40 games worse when he left and then 20 games better when he came back. It's tough to get a good comparison with Bird because he didn't bail on the Celtics at his peak, but he did improve their record by 32 wins as a rookie.