Curiously, just before every new launch there is a spate of "why did they do it this way? it should have been...."
The interesting thing about the Hy6 is that it is a return to the Hassy 500C in size: good lenses up front, good back and the smallest "box" in between. It makes sense that they "off loaded" as much as they could to the side into handle, to lessen the box size. Just like a 500C. While I never cared for the ergonomics of the 500C, the size (with a digital back) is pretty nice.
I for one prefer the viewing style of "top down" and cannot work with the large size of the 645 DSLR's. Too much weight on the wrist, and they are pretty big to hold up to your eye. So welcome is the Hy6.
Regarding the size of the camera - the lenses are about as big as the box, so there isn't a lot to be gained vertically or horizontally in size with a change to the 645 format. Yes, the depth would be less, and the mirror smaller. To address this, they have also concentrated on smoothing that action, successfully according to those who have handled it. So perhaps thats a moot point.
So at the end - we have a new smaller MF camera, open back architecture (to a point), lots of lens compatibility, and sized around the 6x6 - which is a bit odd from the digital viewpoint, but all in all, not a bad thing. Perhaps excessive, and unnecessary - but think of it perhaps as a 645 digital, with the quick option of 6x6 film if needed. Not shabby. Perhaps not "Canon" like thinking - but not terrible either.
And if anyone were to make a 6x6 digital back, well, they'd be in the catbird seat, wouldn't they? I wonder if someone like Sinar is just waiting to see what the demand might be before introducing one. That would be a cute trick. And even if the demand isn't high, how many do you have to sell if you are the only one?