Thank you.Better AF accuracy in one shot by far and wider AF coverage without degradation in AF performance away from the center. I used to have the 5D4 and the 5D3 (backup). Both the 50L and 24L II focused better on the R than on the 5D4. I have since swapped out the 5D3 for the R. So now, the cameras are 1A and 1B. The 5D4 is still better for sports/action, but the 5D3 is better for portraiture because of the better one shot AF. Once the 5D4 killer is released, I'll just have R bodies. Just having R bodies gives you the most flexibility with the least amount of kit because R bodies can use both EF and RF glass.
This is a period of transition. I think a lot more people will transition to the R ecosystem once something better than the 5D4 is released, but if you want to use RF glass now, you need a R body. For now, the 5D4 will also remain as my primary travel camera because of longer battery life and GPS, and I'll only bring EF lenses (16-35/24-70/100-400) on trips for that reason -- the R with be stored in the bag with the EF adaptor on it and serve as a backup.
I can't say that I have had problems with AF accuracy on either my 5D mk3 or my 5D mk4, but I did spend a long time using AFMA to fine tune all my lenses.
They all perform really well now and I would be surprised if I found that a camera from the EOS R stable produced better results. Generally I only use the cross type AF points so maybe that is another factor.
However, I agree with your main point that this is a period of transition, but for now there is nothing that is compelling me to move to the EOS R system.