Where did you see "fast full frame camera"
Why do people always want a fast full frame camera?
That's what the 1D or 7D are for - one a 1,3 crop the other a 1,6 crop.
5-6 fps is not 8-10 fps
I cannot see the point of having a lower resolution FF camera in Canon's lineup.
Many people can see this point.
They do not need huge files, that`s why 12-16mp is enough.
And before someone says noise - the current 7D does quite well with noise, the 1D MK IV is even better, noise is not a significant problem.
Your "someone" is wrong. 7D and 1D MK IV are not good enough.
Not 7D or 1D can produce clean ISO 6400 or even 12800
Adding another camera would be a problem - simply because producing another sensor would be expensive.
This camera would be a dream for wedding/event photography, it will be bestseller ever
There is something called sRAW if you cannot handle a 21MP file - although my 2 years old laptop does fine on them. (2,5GHz Core2Duo, 4GB RAM)
And ISO - there is something called post processing
Nobody on photography on the net is constantly whining for a fast FF camera from Canon.
On that note - 5-6fps, take the 1Ds - it's been around for quite a while.
It's only on here that people whinge about it - and why? Because their Nikon cameras don't resolve enough detail or what? At least I don't see people on a Canon forum calling for a low pixel count FF sensor.
Some people will ask for better dynamic range, maybe the odd one asks for better high ISO performance - but nobody is constantly whinging for a low pixel count FF camera.
It's only here.
And for Nikon users - complain to Nikon - your company decided to take a different approach - don't complain about Canon users liking their detail.
And on performance - the 5D MK II apparently is favoured by a lot of wedding photographers
And to get back to ISO - I'm not sure where you live - but I cannot imagining you needing ISO 12800 for anythng planned like a wedding - it would have to be near total darkness.
Now if you are shooting for news stories going to accidents etc. I can see the point - but anything like a wedding - there is more than enough light for current cameras.
And about selling numbers - I'd doubt it - the extra sensor development costs would make it rather high-priced - possibly in the 1D range would be my guess.