I'm just wondering as to what exactly are the benefits of a mirrorless FF?
One of the most appealing aspects of the full-frame mirrorless (as far as I have read, anyhow) is that you can essentially use any manual full-frame lens ever produced, SLR or rangefinder, via adapter. (Admittedly, you can do this on APS-C as well, but now your hyperfocal marks would all be accurate!)
There won't be any mirror clearance issues with the SLR lenses (like there can be when using such lenses on a DSLR), and there shouldn't be any problems with the various pins, levers, etc., protruding from the backs of those old lenses (due the greater flange distance).
With an EVF, you'd be seeing true DOF without having to swap a focus screen and/or work in Liveview.
Also, these lenses wouldn't suffer from the recently-discussed vignetting/sensor proximity issue that might be a problem for wide or ultra-wide native-mount lenses.
To be sure, a lot of old lenses aren't worth mounting, and others might have problems with color shift and such, but even so....
As a bonus, you could still use all your EF lenses...with EXIF data, aperture control, and autofocus.