I am puzzled over Canon's decision to release two full frame sensors with only 4MPs between them - with the lower rated one going into the flagship and not the 2nd tier camera.
As I see it there could be two very different reasons for that. One bad(ish) and one good(ish).
The bad(ish) option is that the 22MP sensor is not a new sensor at all, but the same sensor found in the 5DII. A technically feasible explanation for that could be that the current 5D sensor has some photosites blocked/disabled for whatever reason Canon decided to do so 4 years ago and now they simply enabled them back on. Another explanation could be that it is in fact the same sensor but with new gapless microlenses. I don't know enough of sensor tech to say whether this is economically or technically feasible in a way that would benefit Canon.
The good(ish) reason could be that as mentioned before, a 22MP sensor allows for a clean 3 x 3 downsample to
1920 x 1080 which would make sense since the 5D represents the pinnacle of DSLR video for Canon.
Agreed. Hence why Canon will not be releasing a 22MP as they would give up too much market share.