On your first bit, sure, but let's say they keep the EF-S mount setup and switch to an EVF. That EVF will gobble up battery and have a very small lag, which are downgrades from an OVF. I'd need to know why they'd shoe-horn in an (almost) must for mirrorless in a body that has room for a mirror. What is the upside to doing that? Cost? More compositional feedback a la LiveView (for the entry level?!). I must be missing something here.
A few random points to maybe fill in the parts you might be missing.
A larger body (for a mirrorless camera) offers the opportunity for a larger or second battery. This point is one of the great Catch-22's of current mirrorless cameras ... the camera is small, so the battery must also be small; yet the camera requires more power, apparently. I say apparently, because the battery of my FUJIFILM X-T1 lasts about 400 actuations; but then I nearly always use the EVF exclusively (and set to auto-on/off).
The EF-S mount allows no-brainer attachment of EF (read L
) lenses. This provides a clear upgrade for a novice from a mirrorless "Rebel" to L
-lenses to a full-frame camera. The EF-M mount is single-use only; plus it will require duplication of lenses for those photographers who want the benefits of using both a full-frame camera and a mirrorless camera.
An EVF is also a cheap way to create a huge selling point ... a big viewfinder. With the small mirror in APS-C cameras it is not (easily) possible to have the same size viewfinder as on for example the 1D-series, without resorting to optics (an expense). But an EVF can be made as large or even larger than the viewfinder in full-frame cameras at basically no extra cost - beyond that of the EVF itself.
Then there is the bugbear of lag ... Hehehehe, the EVF in my X-T1 sometimes does funny things, but overall the lag is ... wait, what lag?
But why should Canon actually do it ... put an EVF in a "Rebel"? Because (a) almost everybody else uses EVF's in their cameras, (b) mirrorless is here to stay and (c) they have to put it somewhere.
On your second bit, you make a fair point -- Rebel can evolve. I just don't think Rebel will evolve overnight. If this rumor is true -- and that's a big if -- one might imagine there would not be a hard exodus from mirrors. So you'd have Rebel SLRs alongside Rebel mirrorless -- they'd have to call them something very clearly different (like Rebel Mirrorless), and that seems a bit of a fragmentation of the brand rather than an evolution. I'm not opposed to it so much as curious why they would do this here instead of grow the EOS-M brand with an EVF, smaller native lenses, etc.
I think mirrors will be a part of Canon's camera strategy for a few more years, especially in their top-end cameras. However, I feel strongly that the 100D/SL1 should have been a mirrorless camera, co-existing with the larger 700D/Rebel "whatever" camera. This would have given consumers a choice between sticking with the traditional or going for the new. And it would have given Canon a great market analysis as to the viability of mirrorless in terms of their brand.
The EOS-M is a cute camera. However, it is a dead-end in terms of long-term development of the photographer, as the EF-M mount doesn't lend itself to a seamless "upgrade" path to ... well, full-frame.
If Canon does indeed decide to grow the EOS-M brand, then they will have to add much more in terms of capabilities and lenses; and this will bite into their top-end cameras. If they don't add these things, then the EOS-M brand remains a once-off, cute camera for hobbiests. (The current poor battery-life of the EOS-M makes it unsuitable as a travel camera, although this would be a great application of it.)