[NAG3LT]

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.