Strange thread ....the D810 replacement is not overdue. The pro and semi-pro cameras normally have a 3 - 4 year lifespan.
What has Nikon's hundred year anniversary got to do with the release of a D850. I can not think of any connection whatsoever.
The D810 was announced in Jun 2014, so it is already three years old. Also, it was not really an all-new camera, it is an upgraded D800 which goes back to early 2012, so the basic model is over 5 years old. On that basis, you could say it is overdue for replacement.
However, I acknowledge the sensor and camera is still one of the best on the market. Digital imaging technology is maturing so the pace of change and improvement is slowing. The market is now smaller since cameras have passed the point of sufficiency for most users. Together this means camera companies may need to keep models in production for longer to recover the production costs, so we may see cameras with a longer life-span than previously.
I think Nikon wanted to have
something significant to announce on the 100th anniversary. In the current upgrade cycle, the D850 was the best fit so that is what was announced. But I agree with BEZ and Ilkka, Nikon should release products when they are ready, and not to meet some anniversary or trade show deadline.