Well, I actully think M is 44 mm, not 47 mm, but maybe just a typo. The thing is - I don't want to adapt an M lens to the RF mount, but vice-versa - adapt an RF lens, to M mount. I know about all those weight/size arguments, but why just remove something, which could an advantage to some. Making M mount 20mm from the start, it might mean a bit larger body, but not so much. Not so much like some of us envision and APS-C RF mount camera.
EF-M has absolutely perfect combo of mount parameters (18/47mm) for Canon APS-C (1.6x crop) image circle. Even Fuji confirmed this, lol. Canon never ever envisioned using EF-M for FF image circle. Only Sony was stupid enough to force their E-mount (18/46.1 mm) into FF service, severely limiting their lens design space. Sony users may well be waiting in vain for FE tilt-shift lenses ;-)
Canon RF mount has best possible combo of mount specs for FF image circle.
Canon EOS M / EF-M lineup is so dirt cheap (relatively speaking) it does not need to be mount compatible with EOS R. anybody able to afford RF glass can easily buy an M50 plus EF-M lens of choice as small&lite travel or backup kit if they want/need it.
"upgrade path" M-RF not needed, it is a totally delusional idea by now.