You can always expand the viewfinder to only show the 1.6 crop area, this way you can nail the focus as you wish but the AF still has the advantage of the expanded area. And as increasingly nailing the focus is going to be down to the camera and not you (especially with fast action) again there's advantage from using FF.Not quite. The smaller the subject is in the viewfinder, the harder it is to nail the focus.
As I said before, with a FF sensor, and everything else being equal, you have more chance of getting something in focus (using autofocus) than having an APS-C sensor.Each had its advantages and disadvantages. Do you take advantage of the extra real estate around your autofocus point to give you more flexibility and a greater chance at framing a moving object? Or do you crop more tightly and give yourself a better chance of getting the subject in focus?
It is true that with a 100mp R you can crop the image in the viewfinder and create what is for most purposes a 1.6 crop sensor camera. But, some on this forum object to paying for a full frame when they will almost always be cropping to at least a 1.6x factor.
The ONLY real disadvantages for using FF over APS-C assuming the sensor is otherwise identical are the potential for faster frame rates due to smaller RAW sizes, smaller end files to work with, and of course, the price differential. But these issues will slowly become less and less important as the tech improves and the pricing differential narrows. And at some point Canon will see there's little point in carrying on with RF APS-C bodies. It may take 5 years, maybe more. But it will happen, I'm sure of it.