I think the 1080 is boutique already. Imho 500$ is a lot of money to spend on a GPU that will fall of significantly when the next generation arrives.
Generally speaking, the Ti series of cards is equivalent to the next generation of cards initial release and thus hold value fairly well for a high end card anyway.
The 780ti is the perfect counter example. At launch, it beat the partner 290s handily. Today, they are on par. Pricing was 350$ vs 700$. That card aged like Charlie Sheen.
Nvidia caught up a little, their architecture is not as far behind as it used to be. But I wouldn't expect those cards to age well either. Not 700$ well at least.
I know you are the enthusiast who likes to stay on the current gen GPUs and who enjoys top end performance and I think you should. But such cards really are not an 'economic' investment (as far as that can be said about gaming hardware anyway).
Edited by Spock, 13 April 2017 - 07:51 PM.