That's true but it would likely entail something like the FE system having to release a new collection of larger lenses to work with such a sensor.
Again as well fixed lens camera seem to have less of an issue when it comes to light angles, I would guess because they can include large rear elements.
If you mean new lenses covering the 36x24mm frame, agreed: although recent lenses for 35mm format SLRs tend to have high exit pupils that make them work well with digital cameras, and so would function fine with an adaptor, I expect at some new native lenses for any new mirrorless mount would be needed to make the system sufficiently marketable.
I also agree that fixed lens digital cameras seem to offer very good opticla performance — due I think ,to the great flexibility in lens design, allowing for example large rear elements that sit very close to the sensor.
My fantasy future lens mount is somewhat shalllow but mote importantly is rather wide, for maximum optical design flexibility. But that goes against the current (and for me slightly misguided) emphasis in mirrorless systems on _body_ compactness as a major selling point.