Originally Posted by Anastigmat
If you move the rear element closer to the film plane, then it creates a new set of problems, most notably vignetting, because the angle of the light hitting the sensor would be more oblique. Therefore doing away with the mirror will not solve any problems. A better solution is to use a fixed pellicle mirror. If the mirror does not swing up, then you can move the rear element closer, if necessary.
The real advantage will be on wide angle lens. That said there is no way to have worse vigneting or anything if the lens manufacturer can, in its discretion, move the rear element as close to the sensor as is desirable…
Eliminating the mirror IS the holy grail of SLR manufacturers (contradiction in terms
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