Quickly....APS-C and "2/3rds" are different sensor sizes.
But, agreed with overall point, I would expect the replacements to the 7DII, 80D, M5/6, Rebels to continue to be APS-C sized sensors.
As for the MP race. I hope that Canon continues to provide the market with cameras in the 20-30 MP range. It would be a shame if that was no longer an option as I agree, I think that is a great MP count for many applications.
But, I like options and am glad to see high MP sensors out there as an option. This forum has several examples of people using the 5Ds(R), but one that I find interesting is a friend (seriously, not me) that has the 1DxII, 5DIV and 5DsR. He is primarily a birder and feels each excels in different instances.
Options are good.