“Really good 4K should have 50/60 fps anyway.
Otherwise the unsharpness caused by motion blurr is compromising the extra sharpness you get from the extra pixels.”
Regarding motion blur, aren’t you confusing frame rate and shutter speed? If you record at 12, 24 or 50 fps, but with the same shutter speed, then there should be no difference in motion blur. You get more jerky movements with lower frame rates, of course, but that’s another matter.
Started out doing photography at the age of 6 using an uncle's old 1940 kodak brownie box camera. At 15 years of age, I decided to buy my very own 1975 Praktica SLR camera. I now shoot with a Nikon D850. I do unpaid TFP and commercial paid work.