Ah... and adding a mirrorless body with a few native lenses to an existing DSLR line up would not be a smart move?
Cheers,
Bernard
Unless you specifically want it for shooting video (which Nikon SLRs don't do well), what does it get you that a second, backup SLR body doesn't.
You don't save money - the new cameras are as expensive as the SLRs they replace.
A second SLR retains the full autofocus capability of your lenses, whereas a Z6/Z7 doesn't. With two SLRs, you can mount one lens on each - say, 24-70 on one and 70-200 on the other - and are able to use them interchangeably, with each one working just as well as the other. You can't do that with mirrorless.
And, if you're dead-set on going mirrorless and gradually building up your mirrorless lens collection, while using the SLR as your primary body, you're free to use any mirrorless system - Sony, Canon or Nikon - since your new lenses won't fit on your SLR lenses anyway, and you'll be able to adapt your old F-mount lenses onto any mirrorless system, at a cost in AF performance. And the E-mount system is a much more proven quantity, with much greater first- and third-party lens and accessory support, than the Z-mount.
Early adopters can do well - eventually. We have seen that with the evolution of E-mount over the past five years. But it takes time, and early adoption is often not the best solution when more mature systems (in this case, Nikon SLR and Sony E-mount) are available already. In five years' time, Z-mount may well be a capable and well-supported system. But that's not a particularly good reason to go out and buy a new system you won't get much benefit out of in the meantime - by the time Z-mount is more mature and supported, the Z6/Z7 will be obsolete anyway, so you would likely have to buy new bodies.
Particularly for the non-dentists for whom replacing a system en masse is not financially viable.
You're going to have to do it, whether you like it or not, if you want to keep taking photos. Your only choice is when you're going to replace them.
In 10 years' time, updates to SLR bodies and lenses will be few and far between, and Canon and Nikon will stop servicing legacy bodies and lenses. They'll still work, up until the point where they physically/electronically break down, but, if anything happens to them, you're on your own.
Sell them now and you can probably get a good price for your gear - switching is cheaper than buying a new system from scratch. Sell them in 5-10 years and you'll get several extra years' use out of your gear, skipping a few generations (which may well save you money), but you won't get much back for your SLR gear at that stage. Either option is viable, and one may be preferable to the other, depending on how you shoot and what gear you have, but not switching isn't a choice.