If you have a 400mm lens you have to be 22m from a 2m lion to fill the FX frame - get 45MP on the lion with a Z9. On DX you can be 33m away and fill the frame - with your hypothetical camera you get 45MP on the lion from further away, or at the same distance you can use a 300mm lens instead of 400mm and save some money. But with an 800mm lens and a Z9 you get 45MP on the lion at 40m. So, if you can be confident of getting within 40m and you have an 800mm lens on your Z9 you don't get a pixels-on-target advantage with DX.
That means you also have to factor in the quality and availability of very long focal length lenses and how well target recognition and tracking AF utilise them. Z has turned this upside down. The AF-S 800/5.6 cost US$16K and sold 3.8K between 2013 and now - effectively up to 2018. The Z 800/6.3 costs US$7000 and has sold 13K since 2022. In Z mount, a lot more people can afford a lot more reach.
Of course, there will always be occasions you can't get close enough, whether because the animal is too small or too elusive, but the pixels-on-target advantage of DX in the D5/D500 days has been eroded in Z mount.
This happened already in F mount with the D850 which could achieve what you describe, and it seems to have outsold the D500 although the latter was less expensive. However, this is a choice that Nikon is making and it's not inevitable. Fuji, for example, is offering 26MP stacked and 40 MP non-stacked APS-C sensors in their cameras which would give some pixels per subject advantage in reach-limited situations over the Z8/Z9. Nikon's solution has been to offer a range of lighter-weight high-quality telephotos including the 400/4.5, 600/6.3, and 800/6.3 as well as the slightly more consumer-oriented 180-600/5.6-6.3. These should give an FX user enough reach in most practical situations, and the aperture of those lenses is intermediate which makes them more portable and affordable than the ultra high-end fast superteles which were for a long time the only options for long-lens photography in the autofocus DSLR era. Thus what was previously achieved with a less expensive DX camera body, can now be achieved by combining an arguably more expensive FX body with a less expensive, but still powerful lens, and an added advantage is that because of the larger field of view and high pixel density of the Z8/Z9, one has more room for post exposure flexibility in framing, though this comes with a smaller subject in the frame in some cases. But one cannot have everything, I guess. One possibility would be for Nikon to implement a framing crop mode which would entail a DX framed viewfinder but the files would contain the full FX data, enabling the subject to be seen and focused on more easily in the center of the frame but then the backup FX frame would be available if needed to avoid wing clips etc. But I guess this would be a too niche use to actually implement.
I think the main issue of making a Z mount equivalent to the D500 is that the fast sensor readout enabling silent photography at high fps and with minimal rolling shutter distortion necessitates an expensive sensor and Nikon might not end up making a profit from such development just for the high end DX crowd. As Fuji has shown, the users might have to choose from fast readout (in fact in their case not that fast compared to the Z8/Z9 in still photography) expensive DX camera with only slightly higher pixel density (26 MP) than the Z8/Z9 and a slow read speed high-resolution high pixel density 40 MP model which would necessitate the continued use of the mechanical shutter for action subjects. What people who are asking for a Z D500 equivalent really want is a substantially higher pixel density sensor with similar read times as the Z8/Z9 and it could end up costing as much as the Z8 easily, and still it would be a niche camera since wide angles would be negatively affected and fast standard zooms would need to be redesigned for DX Z for such camera models to get enough user base beyond the bird photography niche.
The popularity of the 800/6.3 is in line with the other PF lenses and less to do with the Z mount itself; Nikon's F-mount PF lenses were also hugely popular. Nikon is making effective use of PF technology to make long primes more affordable but still good in image quality. Though looking at MTF tests the older AF-S 800/5.6 beats the 800/6.3 PF, so one should not really think that these lenses are truly equivalent. In photographylife's MTF tests the 800/6.3 wide open center has imatest score of 2646 vs. the 800/5.6 at f/5.6 gives 3151. I totally get that the newer lens is a lot easier to use and more practical. I have noticed that in the Finnish Nature Photographer of the Year competition in 2025, 800 mm lenses were used by many, and this is definitely something that was not common in the past.
