I'm way too heavily invested in Sony FE to consider switching now, I'd have to start really hating the Sony FE system, which I don't at present. Another reason to not consider switching is the lack of Zeiss Loxia and Voigtländer lenses for anything else than Sony FE, that's a dealbreaker to me. I played with a Nikon Z6 and I think I would like it better than the Sony A7R2 as a camera, but as said, that won't win me over.
No, there are still too many compromises in the initial Z offerings regardless of upcoming S lenses. The firmware updates were sorely needed and the improved utility is a plus. I'm not interested in external 400Mbs video recording, nor is the AF at the standard of Sony's. the Z's ASIC (processor) is barely powerful enough to support the two cameras. Given today's prices, I'd get more for my money upgrading to an A7RIII. The Sony eco-system and 3rd party support has matured to being the most well-rounded.
The ergonomics is a big factor for some people to try Nikon. But I have no fit issues with Sony full frame bodies.
I agree that ergonomics is a big selling point for Nikon. But I have no issue with Sony on that front, either. I would easily choose Nikon over Canon if I was looking to move, but I am not. I am using nearly all Voigtlander emount lenses now and could not see myself switching anytime soon.
Interestingly, Techart came out with an AF adapter to put emount lenses on Nikon Z. Not easy - only 2mm flange distance difference. I don't know much about it or how well it works. But who would have predicted five years ago people that would want to go the opposite way it was then!
I'm invested in Sony, but I still keep a full Nikon DSLR kit and have been debating going with the Z cameras to consolidate to one brand. I already compared the two systems prior to the new firmware and even then, the Z6 functioned very much like my A7 III.
I've been holding off switching though, mainly due to cheaper native lens options with Sony, but keep with Nikon DSLRs due to some DSLR lenses being cheaper as well (like the 24-120mm f/4 VR, 70-200mm 2.8 VR and 300mm f/4 PF VR).
Also I like using my manual lenses with the Techart Pro AF adapter and Techart's Sony E to Z adapter still needs FW updating to fully utilize the TAP on the Z.
I'll wait to see how next generation pans out with both systems if I decide to make the switch.