I suspect with relatively low pixel count on a full frame sensor and Digic 5+, the 6D will have outstanding image quality and great low light performance. That alone should be reason enough for it to exist.
However I suspect it will still have the typical Canon relatively poor (compared to Nikon) dynamic range. Also I do not understand why Canon can release a camera today that is USB2, non-UHS SD and has such an antiquated autofocus system. If the pricing is $1600 US or less, then it would be more palatable.