In every way other than number of megapixels, the image quality from the D5 will be equal to and in most ways better than the D810 or D750.
How so?
There are only measurable characteristics of an image sensor.
For D4s vs D810:
Resolution - D810 wins, no contest
Noise (i.e. ISO performance) - Draw. D810 wins by a mile at base ISO (due to having a base ISO of 64 vs 100). It's pretty much even from ISO 100-1600. D4s pulls ahead above that. On the other hand, the new Sony 42MP sensor no longer suffers at high ISO, so I'd expect the D810's successor (competing against the D5) to not have that issue. I'm talking about whole-image noise, i.e. printed at the same size.
Colour separation - D810 by a bit. The D4s has a weaker colour filter, due to its optimisation for high-ISO performance. The weaker filter lets in more light and improves high-ISO performance, but at slight cost in colour accuracy
Pattern noise - Draw. Both sensors have negligible pattern noise.
Dynamic range - Draw. D810 wins by a bit below ISO 800. From ISO 800 up, D4s wins by a bit.
When you say less demand on the sensor you constrain yourself merely to MP count.
Because that's the main difference between the two sensors. A sensor only has a few characteristics - resolution, noise, dynamic range and colour accuracy. All of these are interrelated.
The file on the card is the product of more than just the sensor too.
Yes, it's the sensor, the lens, the A/D conversion (really part of the sensor) and the RAW converter. Out of those, every component other than the sensor can be applied equally to the D4s, D810 and D750.
Everything else on the camera - AF, frame rate, exposure metering, etc. - is just there to focus the lens onto the sensor at the right place and time, with the right settings, and has no bearing on the technical quality of the image that comes out the other end.
And even then, not as much MP is necessary as most people believe. 135 film had about 8.5MP.
There's a reason many non-action photographers didn't bother with 135 format film, shooting MF and LF film instead. Generally 645, 6x6 and 6x7 for studio portraits, etc., and 612 and 617 for landscapes.
135 colour film was strictly for action, journalism/events (where you weren't printing large anyway) and amateur use.
The larger the print, the farther the actual viewing distance.
Tell that to the average gallery or buyer.
If you show them a 4x6" print, they'll walk right up to it until it's in their face. If you show them a 32x96" panorama mounted on a wall, they'll still walk right up to it until it's in their face, and expect to see every leaf on the tree.