Canon ef-s body can use EF lenses without an adapter
Canon ef-m body can use EF and EF-S lenses with a Canon adapter
Canon RF body can use EF and EF-s lenses with a Canon adapter
That looks like pretty good compatibility to me.
ef-m lenses can not be used on RF. Well, ef-s lenses can not be used on EF FF. I don't think getting crop lenses to work (in some sort of crop mode) on FF bodies has ever been a priority for Canon. The RF body will take ef-s lenses via an the adapter because there is room for it, so why not toss in compatibility? For my needs, using a crop lens on a FF body is something I would not want to do much anyway.
It has been a trend to slow down on ef-s lens introductions for a long time already. Long lenses will continue to be EF lenses for the indefinite future, as evidenced by the new BWL's. No EF lens -- or EF-s lens, for that matter -- will ever be orphaned -- they will work with all future ef-m and RF bodies (via adapters). Again, pretty good compatibility.
The problem in the midrange is that ef-m lenses so far are designed to be small as possible. What we need for an enthusiast M body is something like a 15-70 f4 ef-m lens of high optical quality. The 15-70 f4 ef-m quasi-L would be big for an ef_m lens, but still more compact than a 24-105 EF or RF lens: just the thing to kit with a future M5mk II or M7 (a mirrorless 7d series). ...Actually, that 15-70 f4 IS quasi-L EF-M lens sounds pretty darn good!
Canon, please steal my idea! (And
DON'T go 3.5-5.6 on us).
This is one EF-M lens that should be designed for performance first, compactness as a secondary consideration. (To go small just use the existing EF-M 15-45 3.5-6.3 IS.)
Only Canon knows what future dslr bodies they will make. I suppose they will make more dslr bodies as long as demand is sufficient. But the end game is to wind up with new ef-m bodies and new RF bodies only. Maybe a decade or more from now the last new dslr will be announced, and it will be a poignant day, as it was when film bodies died. New EF lenses will probably be made for an even longer time, because if the shorter throw of the RF format is not needed it still makes sense to make lenses that can be used by both ef-m and RF camera bodies, as well as the EF and EF-S legacy cameras which will still be out there.
I think all the hand-wringing on this issue is a little bit too melodramatic, but ymmv.