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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

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.

NAG3LT 5y ago
Diffraction is a property of light, it still happens in vacuum. The air turbulence can cause image degradation, but it requires many meters of it to become noticeable and is a complete non issue within tens of millimeters inside the camera. The main reason for the improvements in quality - closer (and for some - larger as well) mount allows more freedom for lens designs, leading to previously impossible approaches. Another change - with photographer never seeing a direct image from lens (only a processed one via EVF or back LCD) - the abberations that can be easily corrected in post can remain less corrected by lens design, allowing better correction of other parameters within the same constraints.