Having a shallower flange distance theoretically allows for an improvement in image quality. The amount of air between the lens and sensor causes a small amount of diffraction since air isn't perfectly clear. The combination of diffraction caused by that gap and a super narrow depth of field is why most camera mounts don't have a lens wider than ~f/1.2. I can't imagine it would make much of a difference at narrower apertures, but there is at least a basis for having better image quality on a mirrorless sensor.

But I totally agree with you, the advances in optical technology and the EVFs make a much bigger difference.