I think it just shows the fact that these cameras (D5, 1D) are extremely niche. They aren't meant for consumers, they aren't even meant for wedding photographers or casual event photographers. They are meant for journalists (able to operate in natural disasters and war zones), sports photographers (where speed is paramount and there is a real possibility of the photographer getting hit by a player), and wildlife photographers going off into the wilderness (although I think the D500's crop makes it better for that role than a D5).
The thing is, while journalists, NGO photographers, and combat-camera-operators (military photographers) need a camera that can survive anything anywhere, they make up a tiny tiny niche. Nikon and Canon have to sell these cameras to a larger market just to be able to sell them at all.
So yes, unless you *know* you need a D5, chances are the D850 is a better camera for you. Heck, I'd say that for many people, the D750 is a better camera (46MP is great, but complete overkill for posting online or print at 8x10 ).
As Dwig and others have pointed out, 35mm in the professional realm was really limited to people who needed mobility over quality. If you could afford the size and weight of a medium format, you used that.
In many many ways crop sensors have replaced 35mm for people who chose 35mm for cost or weight reasons.
Top tier D5/1D cameras have replaced 35mm for people who chose it for robustness and mobility.
>30MP cameras like the D850 have replaced 645 and 6x6 film cameras for people who needed a mix of mobility and quality (Wedding photographers)
33x44 Medium format (Pentax, Fuji, Little Hassy) has replaced the larger medium formats like 6x7 and 6x9 for people who need extremely high levels of quality and the ability to operate at speed (commercial photographers).
40x54 Medium format (PhaseOne, Big Hassy) has replaced sheet film for people who need extremely high levels of quality and speed of operation is not a concern (very high end studio photographers, architecture).
Is this list absolute? No, it's generalities.
It reminds me of the auto industry these days. BMW sells a lot of M5 cars. I doubt many of them see track time, but in many ways either a lesser 5-series (more comfortable) or a smaller M (sportier) would be better for the buyer - but the buyer wants "the best" and BMW needs to sell enough M5s to justify the development and production cost.