Let's look at the numbers.
In the 2012-13 playoffs, the Spurs were carried by four guys -- Kawhi Leonard, Tim Duncan, Tony Parker and Danny Green:
Spur Win Shares 2013 Playoffs: Diaw 0.5, Splitter 1.0, Manu 1.3, Green 2.1, Parker 2.4, Duncan 2.5, Leonard 3.1
If you recall how the Spurs played in 2013, this makes a great deal of sense. Parker regularly drove into 2-3-4 guys and either shot or dished inside to Duncan or outside to the two wings.
Ginobili was terrible. particularly in the Finals, and Splitter played PF alongside Duncan and generally blew (Recall the LeBron block on him in the Finals).
The Heat were able to win in 2013 behind a Herculean effort by James, plus 6 guys contributing:
Heat Win Shares 2013 Playoffs: Chalmers 1.0, Haslem 1.1, Allen 1.5, Wade 1.8, Birdman 1.9, Bosh 2.2, James 5.2
With Splitter playing alongside Duncan, the Heat could get more run out of Haslem and Birdman, both of whom were productive playoff performers. Chalmers was at least OK.
The big change with the Spurs in 2014 is that the Spurs are getting better production out of every big man they play, and Diaw has nearly quadrupled his Win Shares. The team has become far less reliant upon Parker, and move the ball with amazing speed and precision:
Leonard - will end up almost equal in WS
Duncan - already .5 WS ahead, may end up 33% ahead of last year
Splitter - already registered 250% of his last year's win shares
Ginobili - may double last year's win shares
Diaw - 1.9 versus 0.5
Green - will end up roughly the same
Parker 2014 - Minus 1.4 Win Shares
Diaw and Splitter are passing the ball like they were Chris Webber and Brad Miller. Duncan is one-third better and Manu may be twice as good as last year. The Spurs, therefore, have become far less predictable and far more difficult to guard. Using only the guys contributing at least 1 playoff Win Share, the total Win Shares accounted for this year are 15.1 v. 12.9 last year. Not adjusting for games played, that is 23% better play by the Spurs' best guys.
So the Spurs are less predictable, harder to defend, and playing at a level roughly 23% better than last year. How about the Heat?
In 2013, the Heat had 6 guys other than LeBron who had over 1 playoff Win Share. This year, thus far, they have 4 guys other than LeBron.
If you look at guys individually:
Chalmers - terrible, about 50% of the player he was last year. Troublesome for the Heat, because saying Tony Parker is the worst Spurs defender is like saying I am the worst looking of me, Brad Pitt, Channing Tatum, Matthew McConaughy, and George Clooney. If you cannot score on Parker, it is hard to score on the Spurs.
Haslem - non-existent
Allen - will end up roughly the same.
Wade - will end up a little worse
Bosh - slightly better, about enough to offset Wade
Birdman - about 60% as effective
James - slightly worse.
So the Heat has no point guard play, Wade is slightly worse, and Allen is about the same. So their backcourt is in substantial decline. Up front, where last year they got very good production from Birdman and Haslem and Bosh this year they get the same from Bosh and not much else from the other guys.
James has been a little worse. What is clear from his five Finals appearances in 2007-11-12-13-14 is that James' teams need over 5 Playoff Win Shares from him to win the title.
2007 - 3.7 Playoff WS, swept in Finals
2011 - 3.8, lost in 6
2012 - 5.8, Won in 6
2013 - 5.2, Won in 7
2014 - 4.1, in danger of losing in 5
For the Heat to win, they will need to get production out of Chalmers, Birdman and Haslem, continue with some production from Bosh and Allen, and get .3 WS per game out of James. .3 WS in a game is hard. That will require something like 35-10-8 every game. But what the stats tell us is that the Heat simply cannot get enough production anywhere else to win the title. They need a total of 5 Win Shares out of LeBron. There have only been four such seasons ever -- Duncan(title), Dirk(lost to Wade), and LeBron twice.
So it is all up to LeBron. Can he go 35-10-8 for three straight games? If so, I say Heat win. If not, they lose. It is as simple as that.