There is another thing to consider here. I am also a 5D3 owner, and i can say the frame rate is the big limitation, aside from AF and EVF issues. Shooting soccer and martial arts, I've learned the 6fps (at least I think that's what we have on the 5D3) isn't really ideal. If you look at the 1D series, or even the 7D which is the crop action camera, you have gotten double that, which would be much better. So with the R series camera you're getting up to 20FPS. I can only imagine how nice that will be. But of course the limitation is the EFV delay, at least at this point in time. The R5 is supposed to have some killer EVF, so maybe that has mostly been solved? We will see. But it will eventually be solved. Until then we can only compensate with technique. So you'll have to anticipate a bit and hit the shutter slightly early, grab a burst of 20 shots to get what you want. The reality is even in a DSLR if you've waited until you see what you want to capture to hit the shutter, you missed it anyway (there is 1/3 of a second minimum delay between your finger and your brain, plus the mechanical action of your camera). So shooting R5 vs 5D will require a different technique, but not a totally alien one.I think I just found the answer that I was looking for. The article from the pro photographers in Europe (which I had not seen when I sent my inquiry) fully explained the advantage of the DSLR focusing capability and the real-time action vs. the slightly delayed action of the R viewfinder. The only problem is that the article is not really comparing apples to apples. The EOS 1 series is clearly loaded up for a pro sports photographer...which neither the R series nor 5D5 series are geared towards.
Although the same argument about the mirror advantage is still applicable to the 5D series, the fair apples to apples test will be the specs 5D5 vs. the 5R...neither of which are on the market yet. I suspect that, similar to the 5D4 and R, the specs on the new versions will be very similar. And the lag time on the 5R viewfinder will be tightened to be closer to real time vs. its predecessor...advantage 5d5 but closer. Unless either camera includes a dedicated chip to auto-focus ala the 1 series, there will be no advantage to either model.
So instead of jumping on the 5R I might just have to wait until both models are out and see what the jury of buyers has to say and my 5D3 may have to suffice for another year.
I also find it amusing, that a few months ago the theme around here was Canon being too slow in to mirrorless, too slow to 'give up on EOS'...and now so many people speculating both will live forever! Just one of those funny things, not good or bad. My opinion is that it is only a matter of time before technology advances and/or technique catches up and the mirrorless does as well as the DSLR. At that point, we will see the retirement of the EF mount. I suspect that will be within a 2-4 years. Just long enough for them to prove to the DSLR users that they can upgrade without losing the functionality they covet.