Here's something I'd generally be interested in -- but, please, only reply if you have real-world experience, and not if you're only projecting your expectations from interpretations of an unfocused high-ISO macro shot of the inside of soWmebody's lenscap you saw posted on the 'Net.
In what real-world situation does the 5DIII have insufficient image quality for the job but the D800 does have sufficient IQ? That is, when would you put down the 5DIII and pick up the D800 instead of a medium format kit?
I personally can't think of any.
Ask the same question about the 5D Mark II (or any number of other cameras) and you may get the same answer.
Whether the camera does everything you need or not, Canon also has to live up to their customers' expectations. Those expectation are shaped partially by what the competition is doing.
How many people who own a Porsche have ever driven it at full speed around a corner? Does that mean that Porsche should make cars that go slower because for most people "that's fast enough"? Sure, they could. I wonder how sales would look if they did?