Counter-argument:
As you state, Nikon has had significant upgrades every 4 years with a small "refresh" half way.
The in-between upgrades typically have a name which is the same as the camera it replaces followed with an "s".
The D810 breaks the routine in two ways : it is a more important upgrade than the usual mid-term refresh, and its name does not follow the usual pattern (D800s). It was probably chosen to make a distinction from the QC issues tainted D800 name. (like D610 instead of D600 and SB910 instead of SB900).
Still the name "D820" IMO does not convey well the message that we are dealing with successor to the D810 which represents an major update instead of a smaller one : D820 is as much an update to the D810 than the D810 is to the D800. Compare to D4s to D5 : no ambiguity there.
I vote for "D850"

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