Why NIkon is planning a second 400 mm lens (400/4? PF) is a bit doubtful (less so than adding a 28-75mm f/2,8 though)
I think the smaller 400 mm is likely to be PF and f/4.5 (but the shadow shapes can change, as did for the 800 mm, so we can't reliably judge aperture from the (shady ;-)) early roadmap illustrations). If it is indeed f/4.5 and PF then the price could be as low as 4000 €, which is way more affordable than the 15449 € 400/2.8.
Such a price difference can easily mean 5 x as many customers can afford an f/4.5 lens than the f/2.8, just based on price alone. Portability and ease of hand-holding also would be better on an f/4.5 PF than the f/2.8.
In the past Nikon have made 400 mm f/2.8, f/3.5, f/5.6, I do not see why there would not be the need for multiple aperture options in the future as well. Nikon seem quite clever in not duplicating PF lens focal lengths to enable continued sales of both older and newer lenses. For F mount there is 300/4 and 500/5.6; for Z mount there will be 400/4.5 (my guess of the aperture) and 800/6.3. A customer using Z mount can then mix F and Z lenses for a more complete range, if desired, without Nikon having to close the two older options from F-mount users.
the 800/6,3 PF means that there won't be a 800/5,6 soon (if at all). Personnaly I like this 800 mm and the specialized TC800, it sees by far more use thant the 600/4
I think the 800/6.3 PF is a smart choice from Nikon. 800/5.6 is more difficult to transport and use, requiring tripod, whereas it might be possible to hand-hold the 800/6.3 PF and no doubt many will do so (I prefer tripod for long lenses as I would often wait for the subject to do something for a long time). Combined with a 100-400, 500 PF or 400/4.5 PF I imagine those wanting portable long focal lengths with high quality but not the fastest apertures will be satisfied. Although for 800 mm f/6.3 is quite fast (only 1/3 stop slower than the fastest 800 mm lenses).
Over the next decades I am sure Nikon will offer other options for long focal lengths, but having both 800/5.6 and 800/6.3 in the Z lineup at the same time seems perhaps not realistic from a business perspective. Still, Nikon ambassadors have been using the 800 mm f/5.6 their advertising so clearly these photographers and Nikon consider this lens valuable. McNally, Hilmer, and Valberg come to mind. Since it is widely reported that F-mount lenses focus excellently by the Z9, there should not be any strong pressure on a current user of the 800 mm f/5.6 to give up the lens unless they specifically want the portability of the upcoming f/6.3.