...I too can't see a 5th mount. Even 4 mounts makes it hard to imagine Canon's strategy...
Do they see the market needing two mounts? Do they think most EF-S users will go to the M with the exception of 7D users. It seems they want to force the EF-s lens owners to choose M or R. Please help me because I can't read these tea leaves.
My personal view – I am not inclined to buy anything but EF mount lenses.
I rented an R a month ago. I have another job coming up in September where the R would be a good choice (I need something quieter than the 1DxII to pair with my 5DIV. ) I might go ahead and buy an R if there are good deals at the time, or if Canon has announced something better, I may hold off and rent again. Even though my GAS would push me toward an RF lens, it just doesn't make sense for me. I need lenses that can pair with both bodies. I don't want to carry around two 24-105 lenses or two 70-200 lenses (my most used lenses).
Same with any other RF lens. I routinely carry two bodies, one with the 24-105 and one with the 70-200. I need the lenses and bodies to be interchangeable, so that means EF lenses. Fortunately, the adapter works great. Canon has said the EF mount isn't going anywhere. So, as long as I own at least one DSLR body, I will be buying EF lenses.
Unless you are ready to completely drop DSLRs and use only R bodies, I don't think RF lenses are a good investment. I'm not interested in the M series. Maybe if Canon made a 7D style body in the M mount I would consider it, but I doubt that will happen.
Right now, only one mount fits everything and that is EF. I expect that within a few years, if Canon doesn't do it, someone is going to reverse engineer the RF mount and come out with an adapter that lets people mount RF lenses to EF bodies. There are those on this forum that say it's impossible. Maybe they are right, but in my experience those kinds of claims are often superseded by reality. I think Canon may do it, because they may find they have a whole series of cool R lenses that no one buys because they can't use them on their DSLRs.
Just my opinion, but my opinion is the safest route is to stick with EF mount lenses.