The thing is that what you see in a DSLR ocular always has the same (or similar) luminosity as the reality. A screen CANNOT be as luminous and have the same dynamic range as the scene you're shooting.
The thing is, you don't really care about the thing you see through the viewfinder; what you care about is the image that ends up on the sensor.
Prior to taking the shot, DSLRs don't show you the image as the sensor sees it; instead you see the image that the mirror reflects into your eye. But the image on the sensor will be affected by things like ISO (or exposure compensation if you're using that) so you will only see the finished product after the shot is taken.
Mirrorless cameras show you the image that ends up on the sensor, either though the viewfinder or on the LCD screen; and yes, in the past the quality of these screens was lacking, but pick up any mirrorless camera released in the past three years and you're fine.
So maybe I need to try again, but I'm not sold on mirrorless; it seems more like a gimmick to sell new cameras (and force the photographer to change his entire lens lineup AGAIN) while saving money on the simplified internals (while of course raising the prices).
Mirrorless is just awesome. Since you're now getting a great representation of the image you're about to take, EVFs can overlay a bunch of other useful information right on top of that: a live histogram, live zebras, focus peaking etc.
And before you say, well just use Live View on a DSLR, that's kind of the point: to have access to these features you need to get the mirror out of the way. With today's screen resolutions and luminosity, mirrorless is just a superior design.
Another advantage is the shorter flange distance which allows for more compact wide-angle lenses.