[Brightcolours]

I think mechanical strength plays a factor as well.  Imagine coupling a 500/4 to a pro-series MILC, a future 1D-R or whatever.  A 54 mm wide mount will be much stronger than a 35 mm wide mount.  

EF-M was designed for lightweight little cameras.  EF and R are for big and heavy pro gear.

Canon's R-mount is brilliant for its backward compatibility with EF glass.  It'll be interesting to see if Canon puts out an R-mount APSC-sensor camera.  If so I could mount my EF-S lenses as well as my EF lenses (with EF converter), as well as R-mount full-frame glass and possible future R-mount APSC glass.  It'd be the universal camera on which you could mount just about anything.  I see people speculating about an R-mount APSC camera at Canonrumors.

I really enjoyed Klaus' review of the new 50/1.2.  To me the big leap forward here is the lack of (or much reduced) clipping of the bokeh balls.  I imagine the lack of a mirror box and the wide throat diameter are to be thanked for this.  I imagine that larger rear elements, as allowed by the R-mount rather than the M-mount, has its advantages for fast lenses.

Now, what is the future of EF-M?  I have no clue.  But R is a brilliant move by Canon.  Hopefully the power consumption of the viewfinder will be reduced as the R line evolves.

Where to begin...

With big and heavy lenses, you hold the lens. EOS-M has a 47mm throat diameter, not 35mm "wide mount". The EOS-M mount plates have a 57mm diameter.

"Canon's M-mount is brilliant for its backward compatibility with EF glass".

Canon is NOT planning EOS-R APS-C cameras. 

The Canon RF 50mm f1.2 L USM heavily clips bokeh balls. 

Just look at how much is clipped at the edges, and even a bit from the center.
Worse than the EF 50mm f1.2 L USM.
In fact, the RF 50mm f1.2 L USM has about the same cats eye bokeh clipping as my (the) Canon FL 55mm f1.2:

And that FL/FD mount has a throat diameter of 48mm.

The only thing that is better concerning bokeh ball clipping, is that the mirror box does not clip tops or bottoms. Because there is no mirror box.

EF and RF lens mounts have the exact same throat diameter. The difference is the flange distance (20mm for RF, 44mm for EF).

Apparently, the large back element has NO advantage for the bokeh, at all. The big diameter does give freedom for lens design, Canon used it to keep the lens size and weight low, while giving very high performance.

The future of EF-M is clear. As long people keep buying APS-C EOS M, Canon will continue the line. And EOS M does not sell badly at all, especially in Japan. Same goes for EF and EF-S.