Your misunderstand my point. When I said you were conflating two separate things, I referred to your belief that all future Canon mirrorless bodies must use any new mount that is designed for the first generation FF bodies.
Yes, predictions are that Canon will use a new mount for it's first entry into FF mirrorless. But, there is nothing to say that they will use that new mount for all future mirrorless body styles. Their first FF mirrorless cameras may place a premium on size and in order to keep the camera compact, they may introduce a handful of compact lenses. As Fullstop/AvTvM argues, they can keep the lens selection small and keep the lenses compact by limiting the apertures available. If they include in-body stabilization, they can keep the lenses even smaller.
But, my point is that when it comes time (if it ever comes time) to replace the mirror box in the 5D, 1D, 7D and possibly 80D series, those cameras can look essentially identical to today's DSLRs -- retain the ergonomics that long, heavy, fast and ultrawide lens users prefer and for most users function essentially the same as today's DLSRs. When they are ready to make that leap, they can (and I believe will) simply retain the EF lens mount as the native mount on those cameras.
Alternatively, it could be a simple and cost effective approach to offer these higher-end cameras in two different Canon mounts, allowing consumers to choose. Those who have invested heavily in EF mounts would choose the bodies with EF as the native mount. Those who bought into the Canon system with their mirrorless full frame cameras and have purchased several of the new mount lenses, might choose a body that keeps that new mount as the native mount. Far cheaper and less controversial for Canon to go that route than to completely replace their extensive EF mount lineup.