It seems people are using the term high end to mean different things.
Some people define high end in terms of current available performance regardless of where that card sits in the road map for its architecture. By this definition the 1080 is a high end card and will eventually move down to mid range once the 1080ti/Titans are released.
Others define it in terms of where the card sits in its road map. This is the absolute definition and will never change. By this definition, the 1080 is and always will be a mid range card. Similarly the 980ti/TitanX will always be defined as high end cards because they sat at the top of their road map. It won't matter if they bench last in 10 years time, their definition does not change.
Personally, I think the second definition makes more sense