The sony idea interests me, not a fan of pellicle mirrors over the main sensor, I remember the Pansonic L-1 / Digilux 2 / Olympus E-330 had a split system in the viewfinder path, allowing live view on the back and OVF similtaneously.
What if an EVF display was also inserted in the path, with a blind, so that the mirror could stay down for focus performance, but the viewfinder could be switched between optical and electronic?
Although it seemed a convoluted system, and one that would require precise construction (I only mention this because of the issues Canon has had with the 1DX, 1D3 in terms of prism mounting) it did seem to offer all things to all people.
I had a great viewfinder on my now ancient Dimage A2. Or at least it seemed great at the time, I'm sure things have moved on (so potentially even better)
From a video point of view (and I've made it quote clear previously why it's different) it is useful to have a finder that works along similar DR and colour as the recorded image, for monitoring things like bleaching via zebra overlays, focus peaking, moire, scanning issues where tvs or monitors are in shot, audio meters, exposure settings, whether gain is switched on or not etc.
For stills some of these things carry over... Do I want to see what is there in reality, or what my camera sees?
I'm not going to be showing folk the real scene, so maybe I'd rather know how my camera is interpreting the scene so I can adjust to get it a close to my creative vision, or reality, as possible.
A split finder path would give phase detect options for those who need it, and live view for folk who don't.
Maybe thats the way things will go in the future, that OVFs become a specialist feature on the top models only.
I would suggest that for a lot of folks, for most folks outside of pro sports or nature guys, EVF's are there.
Of course whether I'm right or wrong about that specific point, folks will have their own way of working that they prefer, and it's a free market.
I can picture scenarios where a stills guy might work quicker and smarter if they are able to review their images without taking the camera away from their eye. Ergonomically, the position when using an OVF is better for some kinds of stills, now whether that OVF is live view or EVF is another matter..