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.