<lots of good research>
Given Canon's leading position in lens technology over Nikon, I would expect the main battle area for the two big giants in the next few years will be in the sensor area. However, if Canon places in directly a counterpart high MP body to compete with Nikon, and coundn't beat it (which is quite likely from what we can see now), Canon might risk some landsliding loss of market share. If I were Canon, I would try to use some indirect approach, without direct confrontation. One of best candidates to test Canon's new sensor technology is an APS-H body. If it succeeds, then it can be introduced in a FF body. If it fails, it's still a comparison between apple and orange. Besides 7 series is not flagship, any failure in it will not cause any big disaster.
If let me speculate, the most possible megapixel in the possible APS-H 7Ds or 7DX could be around 21MP to 24MP. 21MP is around 36MP in FF in terms of pixel density. 21MP in APS-H has larger pixel size than current 7D, which should naturally provide better IQ given all other factors are kept the same. If any extra improvement can be provided, the new 7 series body is almost for sure will be a success, and that will buy enough time for Canon for its high MP body.
DISCLAIMER: my above take is actually heavily biased.
I am fully captured by the 24-70L II, but my most used focal range is somewhere from 30 to 90mm. That needs an APS-H body to shift the range to my favorite area. Hence, the hope of continuing APS-H line.

;D