The confusion comes from the way you're wording that. You make it sound as if just the difference between FF and APS-C (crop) is what matters to your use case. And of course, that is not the case, as it really is the pixel density that matters.
If you took two images of the same subject, with every setting including distance being the same, one with the 50 MP FF 5Ds and one with the 20 MP APS-C 7D II, you could crop the 50 MP one down to 20 MP and get virtually the same end result as the 7D II. Both cameras have essentially the same pixel density: 20 X 1.6^2 = 51.2. If that small difference concerns you, replace the 5Ds with the 5dsr in the example, which has greater detail due to the canceled low pass filter.
As of right now, it is of course correct that Canons FF bodies significantly lack behind the pixel density of their crop ones. At 32.5, the 90D and M6 II currently set the record and would require a body with 32.5 x 1.6^2 = 83.2 MP or more to be released. Which, when it does eventually, will likely cost more than either of those bodies and also more than an RF crop body with this pixel density would.
Nonetheless, just accounting for the pixel density being the important bit in the wording might help people be less confused.