For me, it will all depend on just how great the technological gains of the D850 are over the D810.
If the technological gains are huge, then it may make sense to go with the D850 at some point; however if the gains are only incremental, or if they don't beneficially impact the work that an individual photographer does, then it may be better to slip one full product cycle and run with the "D8XX" in 3 years time as a more cost/beneficial buying option.
Time will tell.