RF 135 f/1.8

[Fast Glass]

The cameras can do more. Just to add to Jonathan's list: AF spread, AF-eye detect, level/histogram in EVF, more fps, less interference with AF due to mirror movement, ability to shoot 1.6 crop, customization of buttons, etc.

Not to take away from what you said, but the 5Ds could also do 1.6 crop modes. Which is a bit of a misnomer because it doesn't really crop anything and just basically a guide for you to crop in post, if shooting RAW. Which I don't really find super helpful, unless you want to shoot 1.6 crop lenses. But would anyone really do it? Probably not many.

If it gained me a smaller file size for more buffer, faster FPS, 14 bit instead of 12 (A caveat with shooting 20fps on the R5 or R6) then yeah. I could see myself using it, but if I'm still managing full rez images regardless. Not super useful to me.