For the RF f/2.8 trinity, it seems like they designed for maximum compactness and somewhat have ignored the growing demands of their sensor resolutions. But in fairness, I don't think corner-peeping landscape shooters were their target market for these. The good thing about the 15-35, though, is it is much better at 15mm than it is at 35, so for me, it's really a 15-24 with a bonus 24-35. I think the longer range of going from 16mm for EF to 15mm for RF was harder on the image circle than was anything else. No free lunch as they say.
2:1 zoom is 2:1 zoom. Photoshop translation for that would be viewing the image at 200%. Certainly I could resize the image to 90mp in something like Topaz to get a better approximation.
But back to the 15-35, I think you're discounting sensor design too much, placing too much blame on the lens design. I say that because the corner performance of this lens at all focal lengths is actually *better* on the R5 than it is on the R. On the R, 15mm corners at infinity are super soft, and IQ at MFD at 35mm is almost laughable. Both those conditions are night and day better on the R5. Both So Canon did *something* to the sensor design (or to the coatings on the UV/IR cut filter) between the R and R5 that has helped improve IQ. I suppose if they continue these improvements as they scale up to 90mp, we may have nothing to be concerned with.