I have the D850 and it does pretty much everything I want. However, DSLRs (at least Nikon DSLRs) have two 'issues' that mirrorless would solve. Firstly, as good as the AF spread is mirrorless is better. Also, Nikon's liveview is shocking. Add to this the extra benefits of EVF (although far from essential) such as live histogram, peaking, focus zoom, DX crop view, etc then all of a sudden mirrorless seems like the better option. Granted I too like looking through OVF's, but I have to say I was mighty impressed with the EVF of the A7Riii and at times you can be conned into thinking you're looking through an OVF, it was genuinely that good imo. You then get other benefits (for some) such as silent shooting (at all times), no viewfinder blackout in burst mode, higher frame rates, faster shutter speeds, eye tracking and probably a few other things I've missed out.
I feel like we've pretty much reached the limit of what DSLRs can do but there's much more that mirrorless can do, and no doubt eventually do everything better than DLSR. The one big question with mirrorless is ergonomics, can they get it right? Olympus have with the EM1 imo, but obviously they don't have the same big heavy lenses. Can Canikon do it that also work with big lenses? I don't understand why (if the mock up is anything close to being real) why Nikon are changing the layout so radically. What they have now just works.
See my bit above about the latest top of the line EVFs, but as I've mentioned already many mirrorless don't have small lousy controls, not the high end ones anyway.