No doubt the mirror has been around for a long, long time. One wonders if that type of technology is really needed. I played w/ an Olympus MF 4/3 and found that focusing w/ the matched 12-35 (think 24-70) was pretty darn fast.
i certainly haven't done timing tests on focusing speeds, etc. but one ponders if the mirror space was done away with does that offer up any optic benefits? One can always design for a larger effective flange space, not a smaller - somewhere along the line the lens designer had to make a compromise that in a mirrorless could be eliminated.
From what I read, the major barriers to a full transition to mirrorless are:
1. battery life -- if you're a landscape or studio shooter this is not a problem. For events and travel it seems mirrorless needs at least double the battery life
2. EVF lag -- this is pretty close; current best is about 30% more than human visual system lag
3. EVF quality -- a very high-quality EVF is needed for pro work
4. EVF user interface -- zebras; focus peaking; ability to set the tone curve (or selectable tone curves) for the EVF; dials or buttons to allow sliding the tone-curve to check the full range of exposure.
5. EVF low-light quality
#4 may be the easiest technically, but the hardest in practice because it will require a lot of ergonomic testing with real photographers who shoot a variety of styles.
That's my completely uninformed impression/speculation.