In order to build a championship team that's not centered around a superstar or three you need to have a starting unit of 4-5 complete players - players who aren't the best at any one thing, but are very good at everything...or at least the parts of the game that are key to the position they play. The bigs need to be able to defend one-on-one as well as help out by rotating. They need to be able to set solid picks to create space for the guards/wings, and then know where to roll. And they've got to be able to crash the boards. Also, having the vision and ability to pass when lanes open up is highly valuable to an egalitarian offense.
LMA doesn't possess these traits. His only impact on the game is his scoring. And while points are necessary to win, we'd be a far more effective team if we had a PF who contributed more defense, rebounding, passing, and picks even at the expense of points. We can make up for the ~8 points we'll lose by trading LMA for a "lesser" PF by spreading the shots around to players who will take higher percentage or higher value shots.
On offense LMA is a fine (not very good, but fine) #2 option. But he's not effective enough as a scorer to make up for his shortcomings elsewhere for him to ever be a key component to a championship team. His man-defense is fine (again, just fine), but his help/team defense is atrocious. His rebounding improved this year, but still leaves a lot to be desired. I always come back to Bosh with regards to LMA. Bosh has always been a more complete and skilled player. But Bosh was a joke as the #1 option in TOR. Now that he's the 3rd wheel in MIA he's playing in a spot that's more suited to his talents - but MIA could just as easily replace him with a couple players who make less money, and might not be quite the scoring threat but provide better defense and rebounding, and become an even more effective team. Bosh isn't the key to their championship hopes, and if Bosh is better than LMA how are we hanging our championship hopes on LMA?