How about a FF sensor that can do crop? Like Nikon's ...
Say, you are shooting FF, then you switch to APS-C mode and it can shoot like a 7D, with reduced pixels, but the same 1.6X on the focal length. That would certainly satisfy both sides. If you want high FPS, then you'd need the 1DX. Makes sense from a business perspective.
Other than less space wasted what would that serve? 1.6x crop does you nothing by itself for actual effective reach delivered. Of course giant files for small birds in the middle are a waste, true enough.
It might help boost FPS but the Digic 5+ should be way fast enough for at least 6 FPS anyway even at FF and 30MP nevermind 18-21MP.
That's the whole point though. You get the 1.6X reach ... wait ... I haven't owned a crop Canon body before. Does the 1.6X reach have anything to do with how close the lens is to the sensor, or is it merely the sensor size being 1/1.6 times FF?
It is due to the fact that the pixel density is higher. That is, if you get a shot with both FF and APSC (everything else being equal), you get more cropping capabilities with the APSC. The FF would need to have 2.56 times more pixels in order to provide the same cropping capabilities.
The reverse is that the pixels get progressively worse looking (because they get smaller). Ultimately, you get the look of compact cameras (at least if you get a pixel density of around 200...300 MP FF-equivalent).