I'm not sure there's an "exodus" under way but people are definitely switching. Can't wait forever to get the features you want from canon. I have an A7III and 24-70GM on order. I still would prefer to use a canon camera for the friendly user interface, colors and ergonomics but they're not offering the features that I'm looking for. Many people are in the same boat. I may come back to Canon if they ever catch up though.
Exodus ?
Well, I can only speak anecdotally from my own personal experience as I don't have statistics.
I was seriously thinking about switching to Sony, but the YouTube reviews I've seen of the EOS R are enough to persuade me to stick with Canon.
From what I've seen the Viewfinder and AF are excellent.
Sure they don't have IBIS nor non-cropped 4k and that was a disappointment, but right now their lens IS is excellent and I for one mostly shoot stills.
Once they release a model which equals or surpasses the 5D4 in Image Quality then I'm going to buy one.
I just hope its not a 1DX-priced model as I can't afford that ... I just want a mirrorless 5D4 equivalent.