I thought I already replied but don't see the comment.
I don't have a super-sharp memory for this stuff any more but what I think I remember is that the EF MkI was really not sharp at all, but the size, IS and build quality allowed lots of shots especially on travel. The EF MkII was substantially improved image quality but substantially bigger and heavier. Then the RF combined the EF MkI size with the EF MkII image quality.
Well, the R5 is so small and shallow compared to my 1Ds MkIII, I could get excited for an improved EF24-105MkII-sized lens now even though I ultimately never got it on the EF.
Of course you're totally right that higher quality would cost more (and be bigger and heavier). But I'm thinking there might be a market for it. The AF is always nailed, and the high-ISO noise is very low. The IBIS is fantastic. A decade or two ago, my shots were never perfectly focused, were noisy, had hand-shake and subject movement, and I figured that often a better lens wouldn't yield better photos due to these other challenges. Shots might be over- or under-exposed too. But now I can do a burst of a running target and eye focus is pixel-perfect in every shot, the noise is low, and lots of lenses are at their sharpest at 1/2-1/15 sec (eg RF50/1.8). Every pixel is within the DR. Now, EVERYTHING ELSE IS LINED UP and the lens image quality is the weakest link, whereas in say 2000 it wasn't necessarily even one of the top five factors in a fuzzy photo. And while the new lens would surely be bigger, the cameras are so much smaller I think people wouldn't mind. (And the people most concerned about size are using their smart phones.)