Lenses designed for short flange-to-film distances are usually better, true.
But your comment reminds me of the old joke about a physicist describing an elephant. "First, we assume that the elephant is spherical..."
It does
not follow that the
whole imaging chain for short-flange mirrorless systems necessarily produces better final images. That depends on the lens, but also the sensor. Highly telecentric lenses often do substantially better on digital sensors, which generally do a lot better with light hitting the sensor at high angles of incidence.
With film, these issues are generally not as dominant, and rangefinder/mirrorless lenses (at least, WA ones) have real advantages.
Once we go digital the situation is not so clear. The Kodak/Leica compromise -- and make no mistake, it's a compromise -- is to use eccentric microlenses on the M8 and M9 sensors. The result is good, but it is still not clear that an M9 with the best Leica glass is in practice a better imaging system than a hi-density FF DSLR with the best Nikon (e.g., the new 24/1.4) or Canon or ZE glass. At the least, the Leica/Kodak approach reduces the number of photons hitting the sensor, increasing shot noise and decreasing low light performance. It also introduces color shifts at the edges and corners of the image due to the interaction of oblique light rays with coatings and micro-lenses on the sensor.
The M9 and its lenses are more compact, though. I'll grant that much. And rangefinders sure are more fun to use, at least in my opinion.
