Gee, I still have my film using Nikon F2.
forty five years ago I wish I had bought the Nikormat!
The price comparison of the P900 and D810A is totally bogus. The D810A is a very specialized camera designed for astrophotography. The sensor is sensitive to wavelengths that are important for capturing Milky Way and deep-sky photographs. It has ultra-high ISOs. plus it has extended length shutter speeds and an intervalometer that has no relevance to the ordinary photographer.