EF glass performs better on R bodies than it does on native DSLRs. AF off sensor is simply more accurate and flexible than through an OVF. The trade off is ergonomics, depending on the lens.
I can understand why some would be resistant to mirrorless. Perhaps the OVF has already been romanticized, similar to how shooting on film has been. Some will never make the switch, and that's okay. I suspect 5DIV's will continue to function for decades+ into the future and will always be an option for people, whether they shoot professionally or otherwise.
Yes. See Ken Rockwell's use of the EF 100-400mm f/4.5-5.6L IS II USM on an EOS R with both a 1.4x and 2.0x extender (gen 2) in front of it here: https://www.kenrockwell.com/canon/lenses/100-400mm-ii.htm . The R5/R6 will only be better.
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