. . . if Canon and Nikon introduce EVF cameras for pros, they better be sure that:
a. the existing lenses work in the new camera seamlessly and without loss of performance;
b. otherwise, if a new mount needs to be created, and pros need to replace their lenses, all hell will break loose, so to speak. And if the thing works via adapters, then point a above needs to be ensured.
These are good points, but this question is fair too:
Call me dim, but what the hell is it about an EVF which would cause (a) to occur, let alone (b) to become necessary?
The main concern with (a) is that with some mirrorless cameras, their contrast-detect AF system does not work well with lenses designed for phase-detect AF. The issue seems to be that CDAF often requires several rapid changes of direction (""hunting"), so that lenses with heavy, strong ring-style AF motors, excellent at adjusting focus with a single fast movement in one direction, are sluggish with CDAF. (I experience this with Four Thirds SLR lenses adaptor-mounted to a Micro Four Thirds EM5 body.) Thus, CDAF-friendly lenses use linear stepper motors instead.
So long as that is so, getting decent AF with an "EVF camera" does require new lenses as in (b). However, the good news is that more recent advances in on-sensor PDAF, refinements of CDAF, and Panasonic's "depth by defocus" are closing that gap, and both Canon and Nikon are already working on some of this. Canon is even doing innovative things with on-sensor AF in some SLR's, for the sake of their live view and video modes.
Aside: As others have said, it might be that progress in digital still photography will more and more be driven by "trickle down" from video, so tilting the playing field towards EVF cameras.
P. S. I was pleased to see Sean Reid in
his Leica SL review here try to move away from the negative term "mirrorless" toward a more positive naming based in the presence of an EVF, just a day or two after I tried the same thing! But I doubt that his EFC = Electronic Finder Camera will catch on.