Last time I checked, DSLR outsold mirrorless 3:1. Maybe most people are living in the past. I consider buying a Blackberry, you know those ancient phones with a keyboard. Looks good together with the antique D810
Jokes aside..., I know a load of photographers especially in the pro/artist segment (nature travellers, news, reportage, weeding ...).
None of them use a so called mirorless. They almost all have a one, for snaps, chilling and backstage, for saving clicks on their main gear witch is roughly 60% Canon 38% Nikon 2%Hassy/Phase. I speak about around 150 ppl I meet in conventions and other places. Those mirorless are slowly sitting in the car because of social media. Now they use more and more their phone for that.
EVF vs OVF :
I recently broke my D700 (it's not dead but need a repair). I have other cameras at my disposal such as Sigma DP2/3, Leica M9P with summi 50 1.4 Asph + summi 35 1.4 Asph, A7r and some lenses. I do not even want to touch this gear. I'm sad and want my babe back because OVF, weight (seriousness ??).
It is also why I sold my fuji XT-1 + lenses system : just gadget for me.
I might get a D810 very soon if NPS isn't quick enough, and, renting it time to time, it is just the best tool a photographer can dream about in this price range. DSLR is far to be dead, OVF is far to be dead, maybe Focal Planer Shutter will die before mirror (and lenses will be upgraded).
Let's talk about technical revolution in 2020.
Also, D810 is pretty rare in the field. Most D800 users were just spoiled and can't really upgrade. D810 in the real wild are not legion.
The whole A7R2D2 extravaganza just let me cold. Really. Imho, it is not a big step forward over even the D800. A D810 is more serious in all regards (price, build, files and so on...). I think ppl are just very happy to buy new stuff, even if the delta with old gear is minimal and, like the stock market today, the morning glory fall pretty quickly. I'm pragmatic; an A7R2D2 is a kind of cat/lizard/parrot... whatever when a D810/5DIII is a working horse.
When you work, you do not bring your cat with you. If you do, you'll always fear to kill it.