I'm not sure what kinds of subjects you shoot, but I personally tend to shoot at the long to super long tele end for wildlife. For folks like me having an APS-C crop sensor reduces the Field of View giving us a boost on the Equivalent Focal Length (which when one combines what the lens delivers to the sensor and it captures IS the FoV). If one was to blow up the resultant images to the same size the subject will be larger in the frame - that's pretty beaten path stuff. However, the other factor is pixel density. If I was to reduce a FF sensor image to the same FoV as the APS-C one, the number of pixels would be reduced by a factor or about 2.56. So to take the example 45MP output from an R5 FF sensor, cropping it down to the same FoV of an APS-C unit would reduce the pixel count to around17.6Mp - which is not brilliant. Conversely, a 40MP ASP-C sensor (not unreasonable as the 90D had 34MP) would have the same pixel density as 102.4MP FF sensor. So for those of us who DO shoot at the very long end, there is a definite benefit.
Even if Canon came out with a R7 and kept the same lens mount, it would still work well. When the first digital EOS units came out, the EOS D30, D60, and 10D all had that arrangement, it was only from the 20D on that there was an APS-C lens mount. I actually own and still shoot with the first two of those bodies (for the fun and challenge of it) and they work really well with the EF lenses.
The image below is taken with the Canon EOS D30 (3MP) and the EF 17-40 USM, hand-held in available light.
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