In the long run, I expect EVF to completely replace SLRs, even for professionals.
That would require though that the EVF would have the same resolution as the fotosensor. Then the EVF could display exactly the image you'll get in the end. Right now the best EVFs only have 2 Mio resolution and I think thats even counting every pixel three times (once for every color).
The EVF would also need to be fast enough to operate in realtime, with no noticeable delay.
A third condition is that Autofocus systems for mirrorless cameras would have to be performant enough.
Once EVFs are of really high quality, they will only have one disadvantage over SLRs: they will require current.
Otherwise you'll be able to see 100% what picture you'll get - something even SLRs dont offer, where the final image might still look wildly different from what you can see through the optical viewfinder. So, in theory, EVF > SLR.
Also, lenses can be made smaller and more lightweight without the extra distance to the fotosensor because of a mirror.
I think people will accept this trade. They already accepted they need a lot more current for digital cameras, after all, than with film cameras. A bit more for the EVF wont be a dealbreaker, not even for professionals.
My favorite system right now in existence is the Fuji X one. It still is full of problems, but in 1-2 years it might be ready that I will switch to it. The combined OVF+EVF is the best compromise available for a mirrorless system, given the fact that EVF are still far from where they hopefully finally will end up. And unlike all the other systems, it actually focusses strongly on maximum picture quality.