Don't get me wrong. It's an incredible camera, if you don't have a system and you want to buy now, it's probably the one to get (provided you want a dSLR over a mirrorless system). But I think there is basically zero incentive for someone with a D810 to upgrade. Or a D750 for that matter. I think we've out-stripped what is necessary for a while now. And yes while the technology keeps getting better, I think we've been hitting diminishing returns for a while. It's for the hardware obsessed, because I don't think its improvements will make it a noticeably better tool.
On the upside though, if you're on team Nikon there will be a bunch of D810's on the used market for cheap!
Thinking about it? Sure... it'd be nice to get, but as UnknownSouljer said, very little desire to upgrade from my D800. If I was making bank on my hobby, then perhaps I'd be all over the latest and greatest, but as it is, I don't shoot enough to justify a purchase like this right now. The price is great, though.
If I had a spare $3k, I'd probably invest it in glass than a new body.
My time frame for possibly upgrading to a D850 would be about 6-9 months from now. I'd sell my D610 to a friend to help reduce the cost. I'm not so sure now. A D500 has the same pixel density as the D850 and would be perfect for the birding photography I want to dabble in. I'm still kicking myself because I didn't grab the $1800 "D500 + Nikon vertical grip" offer available a few months ago. Seems like a steal now. Probably won't happen until later next year after Nikon feels that everybody pays the +2 fps tax on the D850.