I'm pretty confident that you'll see new EF lenses released for much longer than three years.After having read the Canon whitepaper, I think it provides some real insights into their strategy for producing RF versus EF lenses. In essence, they see the RF mount has having some specific advantages, and they will exploit those for those lenses that can be designed to take advantage. They do a few pages on each of the lenses released, and it becomes quite clear the specific advantages gained by using the RF features (primarily flange distance). And, interestingly, the lenses that don't benefit from those things were released on the same day in EF mount (the 400mm and the 600mm).
What this indicates to me is that they will continue to produce EF lenses where there is no advantage to be gained because both audiences can use them about as well as one another. Thus more sales. Once they come out with 1d and 5d replacements in RF mount, then you might think about a point in time where they will concentrate on RF lenses. But even after initially coming out with pro body models in RF, they're going to sell more lenses by putting them in the mount that is used by both types of cameras.
I feel pretty confident that you'll see EF lenses coming out for another three years or so. The only disadvantage to keeping just EF bodies will be the inability to take advantage of some of the new designs they'll be releasing that can only be done in RF. It'll push everyone in that direction through temptation (Provided they can actually provide an FPS rate during focus-priority servo AF that is higher than 3).
On one hand, there's the simple economics of DSLRs remaining cheaper probably for the foreseeable future. As long as there are $300 Rebel camera kits moving off the shelves, there will be new EF and even EFS lenses. On the opposite end of the spectrum, I just don't see big whites going RF until there is a really convincing 1DX2 alternative on the professional end for sports photographers.