I think in all honesty the savings will be minimal on the current one, but I think the increase will be large on the new one making it look better value. Look at the A7R vs A7Rii. the gap between them was huge and the A7Rii has gone up again as a result of Brex-flation. The discount of the price on the old one wasnt as big as the increase on the new one. It is tough, we are all forecasting, none of us know.
It is only you who can make your decision I am afraid. You know you best.
I would go glass or consider a D500 - which is still a MASSIVE jump from where you are now... if the flagship DX model and will likely last you as long as a D750 would. Afterall, how many omages have you taken and said damn, that full frame sensor would have smashed this image...
You have admitted yourself you are not in the knowledge when it comes to the cameras (Leica toys). You didnt research the 50mm, buying it on the "higher price must be better" logic model - nothing wrong with that, just illustrating it wasnt for a specific photographic need. Now is FX full frame because must be better as it is dearer thinking?
If I look at my shooting, I have a Sony Full Frame and Leica Full Frames. Do I turn out images that demand those camera specifications? no. Am I in danger of bumping up against their limitations anytime soon? no. Could I have made do with a M4/3 Oly and been just as happy? yeah probably...
In terms of total cost of ownership over the next few years, I think we have moved even further into the realms of disposable electronics in the world of photography. We arent going to be adoring our 2009 Canikon D23456 fifty years from now the way some cherish a 1953 Leica III today... Smartphone tech is going to make this scenario even worse so I actually think this 10 year window of your camera and you being partners is optimistic. I think you will be moving more and more to a 3-5 year refresh cycle - you have to otherwise Canikon cannot survive financially. Apple are struggling with iPad sales not because they are losing share, but because users are not refreshing them fast enough. I think this inbuilt obsolescence is going to become more apparent soon.
Whats my point? well just dont sweat it... buy it, embrace it, enjoy it... if in six months you could have got it for $500 cheaper... will you get $80 worth of giggles a month from it between now and then? if you will then it is worth it...
your call. I am not replying to this thread anymore because you are at a mental stalemate with yourself and the only way it will break is if Nikon come on and say "buy it now!"...
good luck... but in the meantime.... post some god damn pictures!!!!