It would really change things up alot IQ would be awesome and the can make the new lenses APS-H and obviously make it to accept EF lenses too.
Since the first mirrorless systems won't be there to replace better dslrs, I guess the ef compatibility will be just the icing on the cake and to ease transition from the eos systems. Canon might use the opportunity of this radical technology change to re-design the lens system, too, so ef will be usable with an adapter and maybe not exploiting all features of the new camera bodies.
The same goes for the sensor size - if the mirrorless systems start at the lower end of the scale, they won't use an aps-h sensor that delivers iq that the targeted customer ground won't max out.
Mirrorless is getting more respected, but it still has some way to go.
Mirrorless will only get more respect if it doesn't only comes in smaller camera bodies or with lower price tags, but delivers features that mirror systems cannot have. And this is on-the-fly image processing with an evf (zebras, focus peaking) or smart af - face detection or imagine your af system being able to track a bird automatically because of image recognition and movement analysis...