I'm not really sure I see higher resolution comp from Canon coming in the near future though, Sony perhaps but I suspect that were now approaching the realms of diminishing returns on 35mm bodies for the vast majority of lenses.
What does seem more likely to me is that the medium format market will take a bit of a bite out of the high resolution FF market as cheaper options are getting more common. Right now I don't think the resolution on Sonys 44x33mm sensor is good enough to draw across large scale business but the larger format does probably have more potential for improvement so say a 70+ MP sensor could be a factor.
The problem with MF is lens selection, particularly at the UWA and long telephoto ends. Most MF bodies and their lenses are designed with advertising, fashion and other commercial photography in mind - you won't get the equivalent of a 14mm, or even 16mm lens in that format, nor will you get something beyond around 200mm equivalent.
A MF sensor doesn't require as sharp a lens as a full-frame sensor to achieve higher resolution, but it's also much harder to make equally-sharp MF optics - the components all have to be larger. Conversely, a full-frame sensor can provide just as much resolution as a MF sensor, but will need sharper lenses to do so. Fortunately, sharp full-frame lenses are much easier to make than equally-sharp MF lenses...
The solution to higher resolution is not necessarily MF, but higher-grade lenses - Otus and Leica-style optics. Currently very expensive, but, with more demand due to higher-resolution sensors, prices will come down, especially if Canon/Nikon/Sony/Sigma get into the game.
Also, deformable curved sensors have the potential to greatly increase the resolution of optical systems by doing away with most of the optical elements in the lens. These are likely to first see use in fixed-lens systems with fixed sensors (e.g. phone cameras and RX-style bodies) but both Canon and Sony have now demonstrated models with adjustable curvature. New lenses would still be needed, though - you could still use a current lens on a deformable sensor, but the curvature would have to be set to zero (making it flat) and you wouldn't get any more out of it than with a fixed, flat sensor.
I think the best response to this for Nikon though is probably to try and maximise there advantages rather than looking to fight a resolution war that will target a market of diminishing size. Basically have a camera that can offer resolution and superior action shooting plus handling.
That looks to be what the D850 is - a general-purpose camera that's not the fastest around, nor the highest-resolution, but has an ideal combination for roles that need both speed and resolution, and an AF system that can handle it. It was what Canon also did with the 5D3, and the 5D4 is a continuation of that (as compared with the 5D2, which was, for its time, a no-holds-barred IQ/resolution camera which barely pretended to have an AF system and shot at less than 4fps).
But Canon released a 5Ds as soon as they had a sensor that could provide the resolution. There's a big market out there for high-resolution bodies. Canon was unable to satisfy that market for a long period of time (and still hasn't made a body with both resolution and high DR, since the 5Ds uses the previous-generation sensor technology), so lost market share in that segment, firstly to the D800, then to the A7r/A7r2.
Personally speaking even as mostly a landscape shooter I do actually find the improvements in AF and FPS over my D800 potentially more interesting for branching out more into wildlife without a massive investment in lenses.
I find it useful for one of the bodies. The other one should be an out-and-out resolution-focused camera. If both have high-end AF systems, with frame rate and resolution the main difference, they could each provide effective backup for the other. And they could both use the same set of lenses.
Sports photographers do the same with speed bodies - a D5/1Dx2/A9 for ultimate speed, another (general-purpose or crop body) for more reach. At present, the D500 would be the 'reach' body of choice for Nikon, but, with an equally-good AF system, the D850 would take that role, due to its greater cropping flexibility.