It all depends on the quality of the electronic viewfinder - with good on-sensor af and if (apart from the fact that a evf draws power) you cannot tell an evf from an ovf ...
That's really not true, for two reasons:
1. You can only add so much gain to the output before the signal you're getting becomes too poor to use even for viewfinder purposes. Therefore, when you're shooting 30 second exposures at high ISO already, unless you're willing to accept a very low frame rate in the EVF, it isn't likely to let you see as well as you could with your naked eye through an OVF.
2. When you're shooting at night, your eye is wide open to compensate for the lack of light. I've never heard of an EVF that didn't put out enough light to cause an afterimage under those circumstances. By contrast, it would be exceptionally rare for an OVF to do so.
Yes, for daytime shooting, there's little difference. For shooting anything in the dark—plays, concerts, starry night skies, etc.—the differences are a bit harder to avoid.
In a DSLR, normal focusing is done by diverting light to a focusing sensor. The camera then focuses accurately to this sensor, not the image sensor. If all is perfectly manufactured and aligned, the image is also focused properly for the image sensor, but as we know, with manufacturing tolerances and wear, this is not always true. aFMA calibration is how we correct for this discrepancy.
And one of the advantages of going mirror less is no more need for AFMA. I, for one, will not miss having to calibrate lenses...... And I can certainly live with F11 autofocus.
Why would there be no more need for AFMA? Did Canon miraculously find a way to eliminate manufacturing variation in lens and body manufacture while I wasn't looking?
In live view, focusing is done on the image sensor so there is no second path to correct for.... Therefore, no mirror, no AFMA.
If that were the only reason for AFMA, then the AFMA value for every lens should be identical (or at least mathematically proportional in a trivially calculable way), because the difference in placement between those two sensors doesn't change when you change lenses. More to the point, Canon would presumably calibrate out that difference at the factory, because there's no good reason not to do so.
If lenses adjusted their focus until infinite precision was achieved, then yes, it would eliminate the need for AFMA. Then again, it would mostly eliminate the need if they did that for DSLRs, too. It would also lengthen the delay before you take a picture, which is why they don't do that. Instead, at least as I understand it, cameras compute the distance to move the lens, and where it stops, that's assumed to be in focus. If that computation is off because the lens is even slightly imperfect in any way, then the lens will consistently either front-focus or back-focus. This is why AFMA varies from lens to lens instead of being a constant value for each body. Thus, mirrorless cameras won't eliminate the need for AFMA, to the best of my understanding.
Please correct me if I'm misunderstanding.