[photonius]

Releasing an APS-C RF-mount camera makes no sense: you would lose the small size of the cameras and lenses. And I expect the production cost would be increased as well: after all, the Nikon Z50 isn't cheap, and it's not where an APS-C entry-level camera should be. Canon does need to support that mount more, but we have to remember that most users that would buy a M50 or M6 will not buy more than a lens or two past the kit zoom, if even that. They already have all the lenses an entry-level camera system really needs, especially since Sigma supports the mount.

EOS-M is here to stay, unlike the Nikon Z-mount APS-C cameras.

Well, I suspect that is the same reasoning Canon is using, i.e APS-C only on EOS-M. But the 600 and 800 mm are clearly aimed at low budget customers. Yet, the lenses were not released for EOS-M. As someone mentioned on the other thread on these lenses, they would be useful on APS-C size for maximum reach.
As to why APS-C RF could make sense:
A) Price point. Presently you can get a APS-C type camera for less than 1000$, including even a kit lens. And you get 24 Mp. To have the same pixel density on a FF you would need a 60 Mp sensor. The best you can get at present is 45 MP (R5) for 3900$. There is still a big price difference APS-C versus FF, especially when looking at pixel density. All the arguments say that this price difference will never go away.
b)   Size. The R bodies can be made quite compact and lightweight. So that is not an issue. But so far we have not seen any lightweight compact UWA lenses. For such lenses, the projected image circle has a big effect on lens size/weight. For tele lenses, the image circle makes not much difference, since the front element determines size/weight of the lens. So, I think it will be difficult to design compact 10-18, 18-55 equivalent lenses for FF.
An APS-C R body, would solve both problems. 


On an EF APC-S body, I can mount the small EF-S lenses, but I can put on also a 100-400 EF, or a Samyang fish-eye. I don't need to carry two systems like with RF and EOS-M.
Yes, of course I can use the old EF, EF-S lenses with both RF and EOS-M with adapter. But if I invest in a new system, I would like to make sure I can get the lenses I want eventually.
At present neither offers me what I want. The RF line-up will grow of course. But the EOS-M, it's too limiting, and it looks it will never get anything like a 600 or 800mm lens. But I don't want to spend 4000$ on a FF body for reach, while not being able to have light weight UWA solutions. 
The EOS-M does look like a dead-end from an engineering perspective (lens communication protocol). An RF-S would give the users the easy solution like there was with EF/EF-S. And I think it is less development work for Canon than to maintain two mounts.