Wowza. Here is where I think the logic fails a bit - if you are putting significantly fewer pixels in a area, this level of crop is simply going to have a lot less data to work with if it came off of a 5D3 - sometimes you need pixels for prints, or certain DPI numbers for publishers, and an APS-C keeps that density higher for longer when cropping in.
The *only* final-IQ benefit to bigger pixels in FLLP (focal-length-limited photography) is when they have less noise per unit of surface area, IOW, a weaker noise spectrum where frequency is measured in mm or microns. This was the case before the 7D2 for Canon DSLRs: recent FF cams vs recent APS-C cameras. I used my 7D for FLLP up to about ISO 200 or 2500, and from there and higher, I preferred the 6D for noise, even though I gave up resolution. With the 7D2, however, there is a 1 stop reduction in high-ISO read noise over the 7D, so the gap is smaller, per unit of sensor area, between the 7D2 and the 3 newest Canon FFs.
Noise aside, talking about optics and resolution, the higher density always gives more detail for FLLP, with the same AA filter strength measured in pixels (with a very sharp lens a higher pixel density with a very weak AA filter could possibly give more aliasing and low-frequency beating).
Higher pixel density (more pixels on subject) allows greater flexibility in geometrical adjustments like resizing, rotating, persective, CA, and lens corrections, etc, without creating artifacts - a completely artifact-free and processing-resistant capture would actually look quite soft at 100% pixel view on a 100 - 200 PPI monitor