You have to wonder, if Canon had released a 5Dmk3 in September (before the 1Dx was announced) and it basically had:
1) generally the same 21M sensor as today
2) the 7D functional feature set (AF system, metering, horizon guide, auto ISO, etc.)
3) a Digic-V processor or dual Digic-IV's like the 7D
4) a slight improvement in the high ISO noise, say 1/2 stop better due to improved sensor manufacturing
5) a 5-6fps shoot rate
6) used the same batteries and grip as the 5Dmk2
7) added a 60 fps video frame rate
8 ) additional firmware options
This for $2499......
How many people would be completely happy with that camera?
You're basically looking at off the shelf technology and mainly adaptation costs involved in the construction and integration, along with improved firmware. Yet what you'd have would kick the crap out of 80% of the market.
I know that for a lot of mid-range, general studio, and wedding shooters, that's a solid camera and offers just about all that is needed in that market segment. When you realize the image quality of a 3 year old 5Dmk2 still beats a vast majority of the competition, these tweaks remove the negatives of the current body and flesh out it's feature set nicely.
Looking at prosumer and up, it would still leave room for a 7Dmk2, an enhanced version of the 7D, same 18M pixels, but improved APS-C sensor, 10 fps. It leaves room for a 5Dx with higher megapixels, and the 1Dx for the pro market.