I think the issue here is that if canon ever makes an APS sensor ILC with an RF mount, older EF-M lenses will not be usable. Unlike current EF users, if this happens EF-M users would not be able to keep using their lenses."It is impossible for this mount to be adapted to the new RF full frame mount"
Why would you want a EF-M lens that only fills up an APS-C area on a full frame mount? EF-S also can't be used on EF and nobody batted an eye yet.
The new mount tho, lets you use EF, EF-S and probably also FD soon.
I see 3 possibilities, none of which are good for EF-M users.
1) canon keeps using EF-M for APS cameras. EF-M users will no longer have the choice of using new full frame lenses (once EF fades out)
2) canon ditches EF-M in favor of RF. EF-M users can’t bring their old lenses to the new cameras.
3) canon makes a new “RF-S” to replace EF-M similar to EF-S. Same down sides as 2.
I think the EOS-M line was a kind of experimental attempt at mirrorless. I think the R line will stick and and it’s just too bad for M users.