From Canon's perspective, RF-S makes sense for several reasons:
- Customers with RF-S APS-C/low-spec bodies can eventually "upgrade" to the expensive L lenses without needing a super-expensive body. Canon can produce cheaper APS-C bodies and lenses to attract entry-level customers, and then keep them as they step-up to more expensive equipment.
- More than just a physical interface, the RF mount enables some features that Canon could never implement on the EF-M system, like storing optical/correction information on the lens, and the control ring.
- As EF lenses leave the market, EF-M will eventually lose some of it's flexibility (since adapting EF lenses to EF-M was a good selling point for the system).
I don't see how the RF-S system could result in a package as small as the EF-M. Maybe Canon will surprise me; or, maybe Canon has decided that the small/travel MILC market isn't worth pursuing?
I personally hope that the EF-M system can live on as Canon's small travel system; there is finally (circa 2018) a good selection of travel-oriented lenses; that was always the big complaint with EF-M system. The EF-M system is a nice cohesive package; the lenses share a similar aesthetic, size, and price. It has a familiar Canon menu system. And it is compatible (via mount adapter) with the large back-catalog of EF lenses; those lenses won't be obsolete for a long time to come.