Interesting though that I think as you move up to FF saving flange distance becomes less of an advantage than getting rid of the prism/mirror/AF sensor. Just look at these new Leica SL lenses and the idea that saving a couple of cms of flange distance as important becomes a bit of a joke. You can shave quite a lot of height off of a body by going mirrorless with an EVF and I would not be at all supprized to see Canon or especially Nikon take that route. Even if they don't attempting lenses is obviously a option.
Exactly. Besides, many people find the a7rII to be too small for proper operation (especially with gloves), especially when combined with larger/brighter lenses. Sony defined a strategy for their imaging division a few years back by looking at their core competences and strengths. They reached the conclusion that they were best at miniaturizing devices and applied this as is to cameras according to the "let's make the smallest possible camera with each sensor size" moto. This works wonders with the RX100 series, which delivers 5DMKIII level DR at base ISO in a pocketable camera.
But when you start to look at FF, one basically non compressible factor is lens size. We are given the impression of smaller size thanks to f4 zoom lenses, but lenses are in the end pretty much the same size at equal specs. The new Leica 24-90 is indeed a perfect example since it manages the feat to be heavier than the new Nikon 24-70 f2.8 VR (described by many in these very parts as a "monster lens belonging to a long gone era" when announced a few months ago)... while being one stop less bright on the long end. And this part is obviously essential for many applications.
Another factor is battery capacity. When factoring in the need to take 1,500 images over a period of 2-3 days without being able to charge (a frequent landscape application for stitchers), the D810 and a7rii end up weighting the same because of the number of additional batteries you must carry along when using the a7rII. I hear that the Leica is superior, but it is also much heavier.
So all in all, the main value of mirrorless cameras are lower cost for camera manufacturers (meaning more margins, not lower prices - the a7rII is significantly more expensive than a D810, even if it much cheaper than the new Leica), the EVF if you like that (with some real advantages and some real downsides), more silent operation and a few more aspects, that are basically non essential to most photographic applications.
Make no mistake, I welcome these new mirrorless developments and think that Nikon/Canon should get on board soon, but I still fail to get the mirrorless over excitement.
Cheers,
Bernard