You're proposing they're selling a 6d-size camera with just the phase af, mirror+metering removed? Sure it's a possibility, but on the long run they'll most likely want to make use of smaller cameras with ff iq.
That's assuming of course that reducing the flange distance would be the only size saving advantage of mirrorless. I would argue that when your dealing with a FF camera the reduced flange distance is less of an advantage than with APSC or m43. With those sensor sizes you dealing with much smaller lenses(and the user base generally will be after less ambitious lenses) so cutting down on the flange distance can produce quite a small overall package. With FF there will be very few lens options that can create a truly small overall package even with a reduced flange distance which will count for less relative to the larger size/length of the lenses involved.
I would say that potentially a more important size saving with a FF mirrorless system maybe the removal of the prism and AF sensor. In that respect the reverse is true compared to APSC and m43, your actually saving a lot more size/weight by removing a much larger prism with FF. Added to that as well of course Canon could produce a camera with duel mounts akin to the current EF-S one that takes both regular DSLR lenses and lenses where some of the optics push into the body of the camera.
Looking at Sony's problems with the FE lenses as well I would question whether very small flange distances are even a good idea. They've looked to try and tie together there FF and ASPC lens lineups but it might well be that the "ideal" FF mirrorless flange distance is more like 30mm meaning a good deal less size saving.