Mirrorless cameras DO NOT compromise quality for portability - mirrors don't improve IQ - period.
Well, mirrors definitely improve the iq of the optical viewfinder vs. a cheap evf :->
It's becoming a moot point - videographers who use DSLRs, don't use the OVF anyway, they get loupes - using the 5D or 7D essentially as a MILC camera. I use a loupe for macro work in live view. If they got rid of the mirror box altogether, they could make a better form factor. Seeing that Canon is emphasizing video and has the dual pixel AF - we may yet get something like that.
I would love a sensibly designed camera with a 3" eye level EVF - could be done. It really makes no sense to have a tiny eye level EVF and a large one on the back - one large eye level one should be fine. We're stuck in the DSLR mindset.
shooting with a long lens, holding thew cam in front of you looking at the live view panel is not the most stable way to shoot!!!! That's why it's designed that way, by holding the camera to your eye you have the perfect balance to get a steady shot. Your elbows basically form a tripod...
Notice too...most video folks also use some kind of harness or a monopod to steady the camera.
So, from a still shooters perspective, it makes perfect sense to have that tiny OVF or EVF. Video has different needs...
I believe the future lies along a different path entirely. I think you give the camera designers of 65 years ago too much credit vis-a-vis the notion that their goal was stability. I don't believe that was the case - I seem to recall reading that their motivation was to avoid the parallax that all rangefinder cameras suffer from.
If you've ever used a Rollei TLR, you would probably agree that they are far more stable (with the neck strap taut and the camera cradled in your hands at waist level) than a SLR held to your face. So perhaps the future is...a digital version of the Rollei 3003 (or Hasselblad 500).
Put a 3-4" retina like display on the top, a full frame sensor (with dual pixel AF) inside, and an EF mount on the front, and you've got a design that would be easy to hold stably, can be used over your head in crowds (like the classic TLR can), and if you really want to use it at eye level, a pentaprism like assembly could attach to the top or back (it could be an EVF or a mirror on the top, on the back it would need to be an EVF). The connector for that optional EVF could feed external monitors (perfect for studio work). External grips could be anything you want.
Most interestingly, the lens mount could be interchangeable. Why not an EF mount, a Nikon mount, a Leica mount, or any other mount you can think of. The flange distance changes as required - the mount that holds the lens mount is the constant - and could contain all the needed electronic connections for any AF lens (or not bother for manual focus).
The basic design could be made by ANY camera manufacturer - in fact, I would almost expect SIGMA (with their Foveon tech) to make such a thing and try to steal some of the business from the other companies.
That's a body I would buy.