Why not
--keep the EF mount for full frame mirrorless cameras
--keep the EF-M mount for those who want a lighter camera/lens package, including those light F6.3 and f5.6 lenses. (in other words, if you want a 70-200 f4 you have to go full frame. You want light, it means light lenses -- and Canon can market the hell out of lightness. You want big lenses, go with a bigger camera). This mount replaces the EF-S cameras/lenses. (In other words, better to piss off EF-S lens owners than EF lens owners)
The flaw in this, I admit, is it means Canon has to maintain a two-lens line.