Originally posted by Rondec if all you are doing is mounting existing F mount lenses on it, with an adapter, you might just as well get a Sony camera or, a D810 for that matter.
Obviously I cannot speak for Nikon, but I am guessing their initial audience is current Nikon users who are thinking of going MILC - much as Pentax might view me if I were younger, or as Canon would have viewed me four years ago. Quite honestly, I would not have come back to Pentax three years if the EOS-M5 had been available four years ago {or if the EOS-M had an EVF}, because I would have viewed it as being superior to both the Rebel I had and the K-30/50 I was looking at. However, with only the Rebel to compete against, the K-30 was a relatively easy choice to make {Sony A6000 was my second choice}.
So, the target audience are people who already have Nikon lenses that do what they want; Z-mount lenses would be better, but these lenses will do. For these people, the new cameras will be superior to both of the options you suggested:
compared to Sony anything - these are Nikon cameras, and Nikon users are just as committed to Nikon as Pentax users are to Pentax {but Nikon users don't whine as much}
compared to D810 - these are users postulated to be leaning towards MILC, which the D810 isn't