I have no doubts that a 1DX or a 7D2 would be capable of capturing 4K video with a firmware update, but unless there is faster storage, you will have slow frame rates and heavy compression. Ask yourself, what is the good of having 4K video that it so heavily compressed that it looks like up sampled 2K video?I would even be willing to bet that Canon could add new 4k features and functionality to the 1D X and 5D III as well, if the time comes that consumers really demand it.
If I were a videographer right now, using either the 1DX or the 5D3 since their respective availability, and Canon releases firmware updates enabling 4K recording with these cameras, then ...
- it would mean that the camera was capable of this functionality right from the start, but Canon chose to "cripple" the camera for some obscure and probably financial reason; and
- I'd have serious doubts about ever using Canon products again.
But, hey, that's just silly old me!
You need fast storage before you will get decent 4K video.
You seem to keep forgetting that cinematic frame rate is 24fps. Anything higher, and you have "high speed recording for slow motion video". It is not standard yet to play back videos on your TV at anything faster than a 24fps video frame rate. The TV REFRESH rate may be higher, may be as high as 240hz, but refresh rate and frame rate are very different things. Hollywood itself is just now starting to EXPERIMENT with 48fps frame rates, however when it comes to any higher frame rates, the purpose is to record at a higher rate that is still played back at the standard...that's what produces slow motion. For all intents and purposes, for consumers, THE frame rate is 24fps. That isn't low, it's just standard. And it's what most people's videos will be played back at.
At 24fps, with standard forms of video compression (which do a remarkable job at not making video look like crap, while reducing it by as much as 72%), Canon's current cameras should be more than capable of handling 24fps 4k video. They just wouldn't be able to do RAW 4k video (however that really is a high end feature, as most consumers wouldn't even know what to do with RAW 4k video, and probably wouldn't have the tools to do anything with it if they did know what it was.)