exactly! agree 100%
no technical reason whatsoever. Any size sensor smaller than and up to 36x24mm could be served by RF mount. Downside would be crop camera bodies and crop lenses that are not as compact as they could be with a smaller mount [like EF-M].
Throughout the DSLR era Canon has used to mount variants to optimize FF and APS-C systems in terms of optical performance and size/weight/price. With RF mount it is clear they have decided to continue a 2-mount approach also for the mirrorfree future.
Sony decided to go 1 mount only and use APS-C E mount also for FF sensor. They and their customers have to live with the consequences of that decision. Amongst other unwanted effects like more complex, heavy and expensive lens deigns I do not think we'll ever see lenses like a Sony FE 85/1.2 or a 28-70/2.0. And even less so a 58/0.95 [although I personally doubt the necessity of such a lens].
In line with their DSLR history [1 mount, F-mount with FX and DX lenses] Nikon seems set to also go with 1 mount ("Z") for mirrorfree as well, but opposed to Sony they would use the large FF-optimized Z-mount also for APS-C cameras. Downside to that approach is that APS-C camera bodies are not possible in a very compact format, due to the physically large mount opening.
There is a clear difference between the EF + EF-S strategy and the EF-M + RF strategy idea proposed by many here though. EF and EF-S mount is exactly the same, the difference is in the optics which for EF-S protrude closer to the sensor. EF-M and RF mounts are different by design and seemingly incompatible with eachother. If the RF mount is going to be canon's main mount, and it seems like it judging by the lenses released, keeping EF-M makes as much sense as keeping the FD mount after EF was introduced.
Historically, there have been few examples of manufacturers supporting more than one mount in the long term. We have Leica M and Leica R but Leica have always been a premium brand which puts it in a very different position relative to Canon. They still discontinued the R mount. We have Fuji, Pentax and Hasselblads various medium format mounts, but again medium format is a premium niche. And Hasselblad still slowly moved away from older mounts.
Z-mount and RF mount will allow for APS-C bodies that are compact enough, as opposed to "very compact". Sure, the smaller EF-M mount and APS-C sensor allows for a slightly smaller total package but then m43 allows for even smaller and compacts can be smaller still.
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