[T]he 5D Mark III does not present such a technological leap[....]
Erm, that's complete bollocks.
Best autofocus system of any camera on the market at the time of its release, best high ISO performance of any camera on the market at the time of its release, best video of any consumer DSLR....
There's a whole hell of a lot more to a camera than just its megapickle count.
Anybody who claims that the 5DIII isn't a big deal isn't a photographer or is just plain clueless.
It may well be the case that the advancements made in the 5DIII aren't relevant to your particular type of photography. And, if all you do is low ISO work of static subjects, you'd be exactly right.
But that's such a marginal part of photography that's already been so well served that it's ludicrous to suggest that that's all that matters.