everyone's going to have a different take on it.
stone, I can see where you're coming from and I agree that there's no reason not to look at the Nikon system once you upgrade to FF.
it isn't reasonable to assume everyone will make that decision, as different people have different priorities. for someone shooting architecture and landscapes, Nikon's AF system means little to me as I only actually ever use the one center AF point. the FPS also doesn't matter, since I've long outgrown trying to shoot HDR brackets handheld (and for that matter, autobracketing isn't important either). however, the 140g difference in weight and 9 MP difference in image size do matter to me.
I'm not sure where the $4000 price tag came from; you can wildly speculate about price all you want but until it comes from Canon it's quite meaningless. I can't imagine a 5D Mark III would suddenly veer upwards with a 30% or 50% increase in MSRP over the 5D Mark II. it could certainly be in the $3000-3500 range, which I think would be acceptable if they shoved the 7D's AF system into it and made some other updates
I agree kubelik,
The more I think about it, turning the 5DIII into a "do it all" camera doesn't make sense to me anymore. Many looking to buy that camera will either buy it for the HD Video or the high MP for landscapes & studio work. People looking at upgrading from other 5D bodies would want more of the same with a few upgrades to bring the camera current. Canon would be foolish to change a successful formula for the 5DIII or price it dramatically higher than the current $2500 price tag.
A new high performance body is what's needed to sit above the 5DIII, and I contend that it doesn't need to cost $4K, not when the competition is doing it for much less....