The dilemma for Canon comes with the high-end lenses. Do they make a 500mm f4 RF lens that will only work on one series of bodies? Do they continue to make the big whites and other specialty lenses as EF series, to maximize compatibility? Or do they design adapters that allow some RF lenses to be used on EF and EF-S bodies?
So long as the 1Dx, 7D and 5D series remain the dominant bodies for sports, birding and wildlife, these lenses probably stay as EF mount.
People compare this to the FD mount, but there are significant differences. Autofocus was always going to make manual focus lenses obsolete anyway, but while the RF mount has some apparent advantages, none are comparable to the difference between manual and autofocus. Mirrorless is different, but not so clearly "better" in all aspects to make users feel compelled to abandon DSLRs (despite what a handful of forum experts might think).
Right now, for anyone planning to continue to use both DSLRs and Mirrorless, the choice between investing in an expensive, specialty lens in EF or RF mount (even if it were available in RF mount) the scales would likely be tipped toward EF or no purchase at all. I have simply suggested that Canon is smart enough to understand this dilemma and is likely considering how to address it. It seems odd to me that only Neuro is willing to discuss this rationally.
I don't think it's much of a dilemna for Canon. They cut off the FD line for the EF line pretty cleanly, even if people didn't like it, they still made it to number one.
They just refreshed a ton of glass for the EF line, so most of those lenses will not likely be seeing another EF varient. Which should be enough to carry them for the next five to ten years since lenses often update at a pretty glacial pace. If canon updated the 7D, 1D, and 5D lines in EF, and ended those lines right there, while working on the RF line of cameras, that'd carry them into the future.
The issue of whether or not to pick of an EF or RF lens should only be an issue if Canon decides to stick with the DSLR lineup.
Even so, all EF glass can be mounted to the RF camera via a standard adapter, so the sensible choice would be to stick with EF unless the lens you want isn't in EF mount, such as the 28-70 f/2.
At least, that's my plan. The 5DMIV easily fits my needs, so unless something happens to it, it'll be my main camera for the next four years. By that time, Canon should have a decent array of RF bodies and perhaps a reason to switch over.