Yep. God Bless Early Adopters.
Let them beta test the new camera and then wait and buy one of the clean, well cared for or little used earlier model cameras they are selling, or sell to places like KEH, at a substantial discount.
If you can buy used from a private seller the savings are usually even better.
I'm not really sure what is needed for most that a d810 doesn't do well, except maybe fps if your that type of shooter, but a 36 mp (or more if replaced maybe?) camera as good as the d810 is probably as good as most will ever need
It's difficult to improve upon the 36-MP models for many people...the camera market is in decline...I really do not think they will rush to update the 810, but the D750 coiuld beocme the D755 or the D770 or the D780, I think. Who really knows? Right now the consumer models sell a LOT of units, and the mid sells fewer, and the 1-digit bodies sell very slowly and in limited numbers.
I say buy what you WANT or NEED, used, whenever you want it. If you buy used bodies, the cost is lower, the depreciation hit is lower, and you can move in or out of the market at will.
Actually now that the D7500 came out, I wouldnt be surprised if Nikon drops the D750 line. I.e. the D760 also loses the second card slot, the battery grip, the AI lenses support.
After all, Nikon never released a D700 successor either.
And they seem to move to higher prices for full quality.
And yes the D810 is a genius product and will be just as good as it is right now, even after Nikon publishes the D820 (or however they will call it).