- Like what you've got: stick with it. D800 and D810 envy is just that: envy.
- Dislike what you've got: stick with it. You have three choices: lose pixels and skyrocket in body quality and frame rate (D5 or Df); keep the same pixels (D750) with exactly the same sensor; or gain a bit in pixels and a bit of body quality (D810). None is sensible change, in my opinion, unless you bought wrong in the first place. Think of this way: the D600 and D610 are a slightly less substantial D3x at one quarter the price.
Special note: a lot of folk have been abandoning their D600/D610’s because of the “dust” issue. Given that Nikon has (mostly) made good on fixing the D600 (to the point of replacing many with the D610), that’s an emotional response rather than a practical one. What’s happened now is that the D610 has become extremely reasonable in price for FX. There’s nothing really wrong with a D610, and the price/performance ratio is extremely low.
The D750 is a step up in terms of features (particularly the swivel LCD), but not really in terms of most performance factors.