For base ISO dynamic range, the A99 II is about 1.4 EV behind the D810 and the A7R II is 0.8 EV behind the D810. (Part of this difference is the difference between ISO 64 ISO and 100, but for low ISO subjects often this difference in sensitivity doesn't matter much; for tripod based landscape you can select ISO 64 unless there is a lot of wind movement effects). At lowest equal ISO settings (nominal 100 for the D810 and A7R II; for the A99 II one has to interpolate to find the matching measured ISO which is maybe 125 to 160), the D810 is about 0.46 EV ahead of the A7R II and about 1.6 EV ahead of the A99 II. In fact the effect of the semitransparent mirror on light loss is evident in DXOMark's "measured vs. manufacturer ISO", about 0.40-0.5 stops. The rest of the differences in the measurements between the two Sony 42MP cameras are likely to be related to high speed reads.
From 3200 to 51200 the D5 is about 1 to 1.3 EV ahead of the A99 II in dynamic range at equal measured ISO settings. For many high fps applications, high ISO is needed because long lenses are limited in f-stop, a fast shutter speed is needed to freeze the movement of the main subject, and the photography may take place in an indoor venue where artificial lights typically require boosting of the weakest (blue) channel leading to stringent test of the dynamic range of the sensor.
Either for fast or slow photography, Nikon has a very competitive solution as it is today (they also have intermediate variants such as the D750, Df). Yes, there will be new cameras in the future. However, Sony's 42MP sensor doesn't really appear to be ahead of the competition in dynamic range, either for high fps high ISO or low to medium fps, low ISO photography. I would expect Nikon to use something else for the D810's successor. If they stay true to the strengths of this line it will be optimized for low ISO image quality and still reasonably good at mid to high ISO. For obvious reasons Nikon should not make a D810 successor with reduced dynamic range at base ISO.