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.