Seriously, some guy just said "very rarely do people shoot in high iso"....are you kidding me?
To be fair, what he said was above ISO 6400. I suspect he's true, only because until the most recent Canon bodies, shooting above ISO 6400 gave unusable results. That's not true anymore. On my 1D X, ISO 6400 is the new ISO 800.
I am in the minority as I never shoot at ISO 6400 (because that ISO setting does not exist in the 5DC).
I know I can buy a camera today that produces wonderful results (for my purposes) at ISO 6400, and I would be thrilled to own one of them (6D, 5D3, D600, D800). I have no desires to have a camera more advanced or feature-rich than the 5D Mark III. I would also be happy with the 6D. My hobby and my disposable income do not allow me to invest in a 5D3 or 6D, even though I know I would be completely content with either one. In my case my upgrade is not limited by what is offered by camera makers, it is limited by budget.
Also, regarding the form factor debate - I would not want a camera with a built-in vertical grip. Personally, I would not care what size the body is for any future cameras (i.e. 5D-class, 7D-class, 60D-class, Rebel-class, SL1-class, mirrorless) so long as the viewfinder is as big and bright as the 5D and the sensor is full frame. I enjoy the handling of my 5D, but I also think the 60D is the perfect mix of handling size and weight.