MVP isn't a measure of value over replacement player. Nash came in and the Suns won over twice as many games. Period. Miami was the fourth seed in the playoffs last year -- they didn't make nearly as big a jump with Shaq as the Suns did with Nash. Plus, the Suns were piss-poor whenever Nash was out of the lineup. Miami didn't suffer quite as bad a drop-off when they were missing Shaq.
You can't look at that unless neither team made any moves besides getting rid of Shaq and Nash. I guarantee you that if the Mavs had stood pat, besides letting Nash go, they would've been in the bottom half of the conference. They went out and got players that made the team better. The Lakers got the wrong players, and never adequately replaced anyone they lost last year -- point guard, power forward, center, head coach. It wasn't just Shaq that left.