What you wrote is broadly true. The real benefits of mirrorless are felt with wider angle designs.
I wonder if the idea that mirrorless helps only wide lenses will go away once we see more RF lenses. It isn't true anyway as the removal of the mirror box already is a big benefit for large aperture lenses of all focal lengths.
Sony has a poor mount and therefore they are a bad example of what can be achieved. And Nikon only just started using a wide mount. Maybe Canon will surprise us, given their vast experience with the EF Mount and its restrictions. At least I don't see them sharing the opinion that the longer lenses won't improve much.
To quote the Canon RF white paper:
"The new RF mount makes possible greater lens design flexibilities:
1. Large diameter rear lens elements that are much closer to the full frame image sensor — enhancing overall optical performance (in particular, tighter control over optical
aberrations at image extremities)
2. Lenses having the same specifications for focal length and maximum aperture as current EF
mount lenses — but having significantly higher image quality — within the same size and weight
3. High optical performance, large aperture (F1.2) prime lenses for full frame cameras
4. Zoom lenses of higher brightness with constant aperture over their focal ranges — while still modest in size and weight
The following section is intended to convey the critical importance of back focus distance and rear lens
diameter on the overall optical performance of a given lens."
It's not just about the
distance of the last element. It is also about its
size - which Canon claims they can make much bigger now.
With such advancements on lens design and the ability to balance a lenses weaknesses with software like they did on the 24-240mm, it seems plausible to say that APS-C cameras will be limited to only the smallest of cameras (M line).
What people have to consider is this:
On EF going from APS-C to FF required a bigger sensor, a bigger mirror, a bigger shutter stronger motors for the mirror and shutter, a bigger optical viewfinder and in most cases a higher quality one.
That's a lot of components that need to scale up and add to the cost. With mirrorless, you can share the same EVF or even drop it completely. There's no larger optical components. There is no mirror and no larger motors. There might not even be a shutter, if Canon thinks they can get away with an eshutter only camera...
If that's the direction Canon is going it makes sense to me to keep the RF mount pure and not muddle it's appeal with compromised crop lenses and APS-C bodies that are incompatible with EF-M and confuse the market.