GHPhotography said:
Finally, shorter flange distance means bending light more to hit the entire sensor, which means lower IQ. As Sony has shown, getting high quality glass on these bodies means none of the size and weight benefits matter anymore, and you are holding a poorly balanced camera with poor/nonexistent grips. It also seems to mean building that adaptor into the lenses, which ruins the whole point of a new mount anyway. Unless you have some new lens design that counters these issues it will always be the case. What magical flange distance means that these issues go away, and what proof do you have for your claims?
This is possibly one of the bigger issues with a shorter flange distance: the illumination of the sensor is from a much more oblique angle. This creates problems in itself with lenses being harder to design while still giving a good image, but also results in significant peripheral illumination fall-off. This is actually already a problem with the EF mount with very fast lenses (think f/1.2).
While Canon may be able to address the illumination issues by developing sensors that can accept light from oblique angles, the cost of building high quality lenses to work with a shorter flange distance may be higher than customers are willing to accept.
A small full frame camera only makes sense in a limited context, as you have pointed out, where you are using focal lengths that are not too long and not too short (possibly somewhere like 50mm to 135mm if you look at the traditional range-finder segment). For most professional photography, for instance, the format of the current EOS bodies probably provides the best ergonomics, so having a mirrorless body is almost academic. While it is a technology of the future, Canon has to satisfy its customer base of professionals, first and foremost.
We need to think about it for a moment: the 1DX / 1DXII are the size they are because that presents the best ergonomics. Canon probably could fit all of the features of the 1DXII into a body the size of a 5DIV (except for battery life), but doesn't because that is not what its customers are looking for.