“At the end of the day it is a still camera with video functions” - Think twice before promoting it like that. Might just be that kind of shortsighted propaganda, which pisses customers off. Nikon did it, Panasonic did it, … “Everything requires some tradeoffs.” For sure that ignorant kind of wise guy talking; - claiming it would not be possible without infringing with the photo-functions,- will cause some major tradeoffs. Are you claiming, that while performing 6 frames per second on a fast moving object your camera does not keep the object in focus? Are you claiming that the canon video cameras do not catch up the focus on moving objects? Your next consumer DSLR will anyway be obliged to carry an Autofocus system for video. How will you promote that? “Not for professionals” Arrogance does not sell!
I think you miss the point of my post, I am not saying auto focus is a bad thing or that its "not for pros". What I am saying is that the engineering of a proper auto focus system would require compromises to the imagining, lens design, and focus algorithms that were all designed to the best performance on a "still" camera.
Lets take your example of shooting 6fps, or heck lets say we are using a 1DX and shooting 12fps. The focus system is design to quickly achieve focus, snap a picture, then achieve a new focus again. Each frame is a new focus event, how quickly or violently it changes focus between these events has no effect on the picture as each is an individual event. Most people will tell you when tracking a moving subject all 12 frames are not always in focus, 1 or 2 might be missed. Think of what this would do to your eye if just 1 out of every 30 frames of video were out of focus, I know it would give me a massive head ache.
The motors in still lenses are not engineered for slow rolling focus that is done in video/film, they are designed to get from one focus point to the next as quickly as possible. This is why some lenses focus faster than others and some bodies focus faster than others. It works amazingly well for still shots. But when you try to do this with video it is not pleasing to the eye, with focus moving too fast and sometimes in a psychotic fashion, video or film require liquid smooth focus changes. In the end it is much more than shooting 30 or 60fps, it is about each frame flowing into the next to make a smooth picture.
I would suggest that people take a look at videos of nikon and sony's DSLRs and how the focus changes when using the auto focus (and real videos not the company produced ones). They may support the feature on paper, but the visual quality in how they change focus is worse than just about any $200 camcorder. Not to mention the noise from the lens changing focus.
Now if Canon wants to bring out a "video" lens and build in a focusing algorithm to support it that would be great, and they may be working towards this with their C line. Now the video lens would be very slow for focusing still shots, but would look beautiful in video. Again there is a reason no one has perfected the still/video + autofocus camera, the trade offs must be weighted. And like I said before Canon choose to design the camera and lenses to be a still camera first and a video camera second. This is is just the nature of the engineering, all political and marketing aside.