I think also when and if the independent lens manufacturers (such as Sigma, Tamron and Tokina) start producing lenses in native Z (for Nikon) and RF (Canon) mounts, it will inject new life in to these systems. As mentioned above, if companies like Nikon and Canon are scaling back their R&D investment, then despite the cracking start Nikon has already had and will have in the short term with the available native Z lenses, if somehow Sigma and Tamron could not only start filling in the blanks but perhaps also offering cheaper versions of the classics (say 24-70 F2.8, 70-200 F2.8) in a smaller, lighter package, I'm sure there would be a lot more interest in the system.
As it stands (and as I understand it), Nikon isn't very forthcoming with data to allow these manufacturers to do that (and some would argue why should they with the millions they have no doubt already invested), so unless it's relatively straightforward to reverse engineer the tech, and that these manufactures can actually see a viable market for such lenses to make it worth their while, then I don't think unfortunately it's going to happen anytime soon. Whilst the FTZ adapter can of course currently be used to adapt existing 3rd party lenses to the Z format, a lot of people are quite averse to "adapting" lenses, and it is an additional piece of equipment to have to carry with you (or not forget) ?