They are worth what people are willing to pay. A new card will drop prices, but they will not make 980Ti worthless.
The 1080 is not THAT much faster it would seem.
I think it's more the point that the 1070 is also faster (so Nvidia say).
Also let's not forget how much mature drivers change things.
The GTX 680 was 30% faster than the 580 at launch, but 40%+ by the time the GTX 780 came out.