The whole point of the Z9 is that it achieved auto-focus parity with the D6 for sports and wildlife, it was really obvious that the D6 would be the last D camera, just like the F6 was obviously the last F camera. The re-designed ultra-teles (as opposed to "take existing optical and mechanical design of the EF version and integrate a flange extension" like Canon did) slot into this strategy by offering a real benefit over the preceding F lenses.
Nikon has a pretty long history (in Western memory going back to the Nikon S) of executing well and consistently. They're often not the first mover: Nikon S was a "best of" Zeiss-Leica body (pretty much all Zeiss, but using the Leitz shutter - this is why Nikon lenses mount, focus and zoom to the left), became quite popular. Nikon F wasn't the first SLR system, either, but well executed and became the benchmark.
Nikon has a pretty long history (in Western memory going back to the Nikon S) of executing well and consistently. They're often not the first mover: Nikon S was a "best of" Zeiss-Leica body (pretty much all Zeiss, but using the Leitz shutter - this is why Nikon lenses mount, focus and zoom to the left), became quite popular. Nikon F wasn't the first SLR system, either, but well executed and became the benchmark.