That only seems relevant if owning all five of those cameras — and carrying several of them in some situations — is a preferable alternative. [Management consultant hat on] Market viability often relates to Pareto-optimality: roughly, having a better balance of pros over cons compared to each single alternative for the priority weightings of a sufficient number of potential customers.
You don't need to own all five cameras - just the right one for your particular situation.
Own a lot of F-mount lenses? What's the incentive for moving to Z-mount now, as opposed to sticking with D850/D5, allowing your lenses to age out/break/get stolen instead of buying more, and choosing a new system in 5-10 years time based on the situation then?
Shoot Canon? If you're happy with the base-ISO DR (and you presumably are, since you're still using it) what's wrong with sticking with Canon SLR and seeing what Canon releases on the mirrorless front in a few months time? Or, if you want better IQ but have a large collection of Canon lenses, why not the A7r3 with the performance-proven Metabones adapters? What does the Z7 offer you that each of these other options doesn't?
Don't shoot full frame and want to move into it? Why the Z6/Z7, with minimal initial lens selection and only a promise of future performance and future lenses, instead of the much more mature E-mount system with known - and good performance?
At the moment, there seems little logical reason to get the Z-mount, except as a compact secondary body for current F-mount users for when they don't want to carry a large body around, or for shooting video. This will almost certainly change with time, as Nikon releases more Z-mount lenses and reduces support for F-mount. But, until that happens, it just doesn't look particularly attractive as a primary system.