Camera-size means also includes lens size. So for the image quality (and a shallow depth of field) a big sensor is needed, which means big lenses.
I shoot advertising -- I couldn't care less about paper thin DOF. Most of what I do is shot between f5.6 and f16. YMMV.
Just look at a NEX with a telephoto lens, the size of the body is becoming less important, because it's the lens that makes up the bulk. Then there is the handling issue for serious work with a camera. I think most people would get a cramp in their hands, if they have to cover a sport event for 2 hours with a NEX+telephoto-lens.
Always use the right tool or the job. I don't think many people will be covering the Super Bowl with a Sony NEX.
On the other hand, these systems are nice to go hiking ...
They are also great if you are holding a camera for 8 hours during a commercial shoot. Some Pros are talking about dumping their DSLRs for mirrorless. My primary lenses are 24mm and 85mm primes. A NEX 7 and a 24mm f1.8 Zeiss weight about 20 Oz total.
People who do not care that much about IQ, will buy a smaller camera ...
People who don't care will use their phones. The P&S market is dying. And not many people will "step up" from Smart Phones to real cameras.
I think there is room for an APS-C or even FF EVIL-camera in the rangefinder style. SONY is close to this, but the ergonomics of the NEX sucks.
Since I got my NEX 5n my Canon DSLRs don't get much use. When I pick-up a NEX 7 I doubt they will get any use.
If I were Canon, I would take some time to find the ideal sensor-size for a future EVIL-System. It is not an easy task...
APS-C is the perfect size. Canon needs to get their act together and make a small and light Pro quality camera. It doesn't need to be mirrorless, a Digital EOS IX (an APS film camer from the 1990s) would be great with a EF 85mm f1.8. or a new 22mm f1.8 EF-S.