When analyzing a lens prescription, it will be evident that not every lens element is a good candidate for autofocus movement. A lens element in one of the actual L-mount lenses weighs 3.8 grams, and has a focus movement of 640 steps controlled by a stepper motor. A comparable lens element that might be suitable for AF movement is the actual Summicron 2/28 mm ASPH which weighs 47 grams. Not only is this weight too high, but there is also not enough space in the mount to accommodate the required movement and the motor, however small it is. The 640 steps explain why AF is more accurate than manual focusing and much faster.
Most Leica users however take pictures of stationary subjects. AF is hardly needed in these situations.
A new generation of lenses would be needed for incorporating AF in the M camera. The M lenses have already grown to a significant volume. The quest for wide apertures and high performance has a price already. This balancing might be the reason why L-mount lenses are limited in aperture. The size of the diameter of the L-mount would not be a hindrance. Size and performance are in an inversely proportional relationship.
The performance parameters for actual Leica lenses are related to the MTF curves. This is good for the important characteristics that define image clarity: edge contrast (often referred to as micro-contrast) and resolution. The price to be paid is the uniformity of the reproduction.
The overall image quality is the result of a balance of aberrations. High MTF values imply that the residual aberrations are very well corrected with a smart balance of fifth-order aberrations. The reverse of this strategy is a uniform image quality. It is comparable to the sharpening algorithms in post processing software. They all function in the same way and the results are interchangeable.
The older generations of optical designers had less options to work with and used every trick they knew to get a decent result. These lenses were less good, optically speaking, but they showed a character that modern lenses do not have.