Add fast moving subjects and even at f/2.8 or wider I find myself pushing the development past ISO 10000 equivalent.
FWIW, using Topaz software I often get what I consider astounding results with my D500 which does a better job of focusing than my D800 and can make a bigger difference in the final result.
For instance, here is a photo I did last year with my D500 for a friend (not taken with a "full-frame" camera, but I wanted to show the full frame capture prior to processing):
As you can see, in addition to pushing the conversion of a shot taken at ISO 6400, there was cropping too, so the equivalent ISO is well above 10000.
As for the D700 being as good as the newer, higher pixel density cameras at high ISOs, that's not really true. If you are viewing both at 100% then you could argue that, but if you view the higher pixel density file at the same effective viewing distance as the lower pixel density file then the former is a noticeable improvement over the latter.
The point I'm trying to make here is that there's nothing wrong with the high ISO capability of the D810, and (FWIW) I get by quite well (IMO) with my D500 under reasonably challenging lighting.