People are getting a little carried away, I think.
The camera system won't be void of mechanical components. Focusing, zooming, image stabilization (both in body and in lens) involve moving parts, and some of the controls also operate mechanically. Not that I think there is any great need to get rid of mechanical systems, IMO. Of course, high fps rates and video operate with electronic shutter and this is an important development, to have electronic shutter with fast enough read cycle so that most rolling shutter and banding are avoided. However, there is likely some cost in dynamic range (dpreview estimated less than 1 stop compared to Z7 II, which in turn is slightly worse than the D850). This is still better than cameras like the D6 which have a sensor optimized for a narrower set of applications, but it may not be the best DR achievable for applications such as landscape. High fps rate and fast read time also may be incompatible with the highest resolutions that might be implemented in the future, such as 100 or 200 MP. Electronic shutter does have advantages for high-resolution photography as shutter-induced shake is avoided (but most of this is already avoided using EFCS).
EVF lag is dependent on light level as in dim light they need to amplify the signal and e.g. to see objects at night, they may also need to collect data over a longer integration interval further increasing the delay. Variable delay can be a problem when timing shots in different lighting conditions. I believe a lag-free EVF when I see it; currently I don't really believe it's possible. I know my timing goes completely off when photographing with an EVF camera in a dim restaurant, the captured shots do not match what I saw in the viewfinder. Z9 may be better, but whether it is good enough, remains to be seen.
Low light creates another problem, for the AF. In low light, autofocus based on embedded PDAF sensors can struggle and the cameras typically augment the data with CDAF or switch completely to a CDAF algorithm, which can work poorly with adapted F-mount lenses (apart from the stepper motor lenses). Although it has been shown that adapted lenses work great on the Z9 in bright daylight, I haven't seen a similar demonstration of the capability in low light (with adapted AF-S Nikkor lenses). Time will tell.
IMO silent photography is the big benefit of mirrorless cameras and the Z9 appears to feature it in general photography without significant artifacts (beyond those that the mechanical shutter also produces). However, currently the full-frame stacked BSI sensors are not available on any medium or lower-cost cameras, which suggests that the cost of producing such sensors is too high (or the production volume that is possible now is low) to use in the mid- and lower-end models that probably constitute way over 90% of the ILC market. Thus it remains a dream and not reality for most photographers, including professionals except the more wealthy end of the market.
One should probably factor in the cost of a Z lens setup and backup bodies etc. into the cost of gaining access to the full capability of the Z9. So if one really believes that it makes sense to do a broad variety of subjects, the cost of the "transition" will be very high. It will take quite a while to happen for most photographers with existing complete systems. Many lens types are still absent from the Z lineup, including all f/1.4 lenses, 135mm, 200mm and 300mm primes, tilt/shift lenses, and also movie-specific lenses including powered zooms (that other manufacturers such as Sony and Canon have). The 16mm flange-back distance may be limiting for the implementation of built-in ND filters (not sure, but think it might become a struggle) which are needed for video in bright sunlight and may be helpful in other circumstances as well.
While I like high-resolution still cameras, I don't believe that high resolution is genuinely useful or often practical in video. Generally, to create fluid movement, a slow shutter speed is needed and this blurs the details in the main subject. It's of course possible to have high resolution landscape video etc. but I generally feel video is best used for moving and living subjects and high-resolution is best used in still photography (because to perceive details, one needs time to scan the scene or image, and in video things are in motion and so there is no time for details). I can understand why Nikon would argue that high-resolution in video is just as important as in stills because their main argument for their system is image quality. But I believe that for most professional applications of video, even 4K is overkill and probably most work continues in HD, certainly in the case of streaming. What disadvantage does an 8K camera have for HD? Usually the HD in high-resolution cameras is not as high quality as it from dedicated professional video cameras that are optimized for it. You can get a good outcome by recording in higher resolution and exporting in HD but this isn't applicable to applications requiring immediacy. Z9 has no physical audio controls, only one microphone input, and no built-in NDs. The colour profiles are different from what more traditional video camera companies use which means you can't as easily collaborate with others on video (especially for live coverage). I just don't have such a rosy outlook on this. I am not doubting that it's necessary for Nikon to produce equipment for video but don't really think that they have the right plan on how to do this; I think it's much more important to have access to lenses designed for video (which Nikon are doing to some extent by considering focus breathing etc., but not yet power zooms) and full controls on the camera (designed around the needs of video work including comprehensive ND and audio controls) rather than 8K. (I totally get that because of the high resolution of the Z lenses Nikon would want to argue that 8K is the way forward, but I think it'll be difficult to get people on board).
I also don't believe that photographers equipped to do stills will be skilled in video and vice versa, despite the claims that these media will integrate into one and you just need the right camera (!). The processes are completely different and most of the equipment are not shared. Video requires gimbals, fluid heads, cranes, powered zooms, microphones, audio recording systems, streaming equipment, GPUs, and above all a plan for everything when shooting, and often a crew. In still photography, a single photographer can capture shots from different camera angles by quickly moving without having to consider continuity which would be a kind of straightjacket (that videographers have to adhere to, but still photographers have more freedom). Typically to cover multiple camera angles in video you need multiple cameras and operators. Post processing is very time-consuming as well, the audio needs to be cleaned up and artifacts removed, colour needs to be maintained across the cameras and over time, and it's more difficult to edits since one has to consider the continuity there as well. I just don't believe there are great advantages to the hybridization. Believing that it's a good idea leads more people to invest in the wrong equipment and waste money instead of getting the right equipment for the tasks and also limiting one's focus is necessary to acquire the required skills.
I don't believe that it's a good idea for most photographers to try to be generalists. It can be more difficult to market work as a photographer if one shows a lot of different disciplines of work, as people are usually looking for those who do specifically the kind of work that the customer needs in a particular job and may not believe that one person can be as good in photographing and videographing every subject as a specialist.
I have no doubt the Z9 will be a great camera just don't believe that cameras in the future will be all electronic or all-purpose instruments and photographers will be even less so.
