This single card slot is a complete red herring. Ive shot 79,000 shots on a Canon 6D, 42,000 on a Canon 6D MKII and around 8,000 on the EOS R. Ive shot around 50,000 on a 5DS in every card case and with multiple SD cards (I rarely use the CF card in the 5DS) Ive never had a failure but more importantly never risked taking one camera body to an important shoot. Ive had the mode dial go wrong on the 6D MKII and the remote socket on the 6D all reasons to have to swap out the body because of what I was trying to shoot.
As to your comments about "not being a professional camera in the R line-up". Well we have 20 EOS R cameras in rental with to date 64% utilisation which allowing for turn-arounds is pretty high, equal to the 5D MKIV and above the 1D X MKII. They are mainly being used as second cameras by Pros (as are the 5D MKIV) but still have plenty of use so in my mind that makes it a "professional tool". Ive a friend with a 5D MKIV and a 6D MKII who is a wedding photographer both cameras get equal use and he doesn't fret about a single card slot on the 6D MKII.
Not everyone who makes a living from photography is a sports photographer, or shoots wild life or fast jets. The majority of professional photography is fashion, portraits, products & food most of which doesn't need a fast frame rate.
The EOS R is not perfect (Ive yet to find a pro that likes the Touch Bar & prefers the toggle switch), but the images are equal to those from the 5D MKIV in a much lighter package. Now if you were to mention the electronic viewfinder as opposed to optical I can think of a number of situations where optical is superior and where when Canon get to do a R version of a 5D MKIV, 5DS, 1D X MKII they will need to improve or eliminate the lag when you take a shot in the viewfinder.