The only FF "mirrorless"exists is LEICA M9. It is a $9K camera. Let us look back into history. In the late 60's the price of the best Canon Range finder camera is about 80% of Leica M3. So if we strap the curve, A Canon M9 comparable mirror from Canon will be about $6k to $7K. Will you buy a 1D-X or the Mirrorless. Also Leica may be holding the offset micro lenses pattern also. Event some Leica lense cannot be used in M9 due to incident angle concern. If the FF mirrorless can be built so easily and cheaply,whay nobody is doing it???
I am not saying a good mirrorless cannot be done. It just have to be compromised and canot satisfy every body.
IMO a full frame mirrorless might be offered by canon for roughly 1200 Euro/$ if based on 5D Mark II components: It's just a exercise in removing non-needed parts:
Mirrorbox with mechanics and motor, pentaprism and related optics, phase detect AF with mechanics.
This reduces the need to align different delicate optical parts with an internal chassis - can be reduced to "hold sensor and lens flange".
Add a 1024x768 pixel EVF with good optics and you will have a 1200 Euro/$ FF mirrorless camera.
IMHO this camera might not be attractive enough to the typical FF user who needs fps and fast AF. On the other side for landscape, macro photographers and videographers such a system might be extremely interesting.
Your argument with the offset microlenses is a good one but I think it is just important for Leica because they never had a mirror box for their M-series cameras and lenses - lens back elements have a shorter distance to the sensor and the incident angles are larger (compared to a perpendicular axis of the sensor).
EF lenses are designed to bend the light at smaller angles - to allow the mirrorbox gap between last lense element and sensor. Again IMHO: This should avoid to use offset microlenses on the sensor.
I agree with you that comparable compactness (with 35mm film) is not possible due to the need for batteries and other stuff. Additionally the waste heat of the sensor hast to be spreaded - perhaps a special problem for a FF mirrorless of compact size to keep the sensor at low noise levels. Camera size helps here a lot: To give enough material to spread the heat and enough surface to remove it from the camera body!
Best - Michael