For me, the major issues I consider when buying new equipment are the weight, cost, and flexibility in relation to potential image quality.
For example, if I'm considering the purchase of the latest camera body of a particular brand and format for which I already own lenses, such as Nikon, then, as I understand, any increase in pixel count will tend to increase the 'effective' resolution of all my lenses, in the sense that the higher pixel-count image, when down-sampled to the smaller size of my previous, lower pixel-count camera, will be sharper. All my lenses will have effectively been upgraded, at least to some slight degree, depending on the percentage increase in the pixel-count of the new camera.
That's just one aspect to consider, which on its own, would not be enough to justify the purchase, unless the individual pixel quality had also improved to a noticeable degree.
Justifying an upgrade to the Nikon Z7 will depend on one's current equipment status. Those who already own a D850 will have less reason to upgrade than I have, because I'd be upgrading from a D810, and the additional features of the Z7 seem to incorporate all the benefits of the D850, such as a BSI 45mp sensor.
Checking DXOMark's results, I see that the D850 already has close to a one stop DR advantage over the D810 at ISO 1600, and a 1/2 stop advantage at ISO 200. I don't know if the Z7 will improve on that, but it's unlikely to be worse.
Another obvious benefit of an upgrade to the Z7, from my perspective, is the addition of state-of-the-art image stabilization to all my current Nikon lenses (using the Nikon adapter). Those lenses that don't have any stabilization, such as my Nikkor 14-24/F2.8, would have up to 5 stops IS. That's remarkable. It means I would be able to get a sharp image at 14mm, hand-held, using a shutter speed as low as 1 sec.
As I understand, those Nikkor lenses that already have built-in VR will get improved VR when the lenses are used in conjunction with the IBIS of the Z7.
The above mentioned features alone would be sufficient justification for me to upgrade to the Z7. However, there are also other features which I might find useful, not least of which is the improved Video and 4K capability, and the availability of future lenses which should be both lighter and sharper, especially towards the edges.