I already sold my rigs but it still profitable in my place, the sole reason I sold it is I already got my ROI and rumors about new GPU coming from Nvidia. sold it early before GPU price plummets for me is the best idea.
The GPU prices of the currently available GPUs may go down with the release of a new generation, but that won't change the performance of the cards you already have. My 1080 Tis will still kick ass.

New gpu's that are capable of higher hash rates will drive up difficulty faster than it would be driven up if a specific network hash rate just stayed the same with current hardware. Difficulty increase will result in older cards being less profitable to mine with. That's not to say that I would be selling off my 1080ti's right now. I don't see myself doing that in the near future. I think that in the near future, I will sell the 1070ti's that I have though. I'd like to have some capital to replace those with newer gen gpu's.
The other thing I'm thinking about is using the capital from the sale of my 1070ti's to buy Vega's. We're not seeing anti-asic fork support from anyone at the moment other than those coins that are better mined with Vega's.
I'll never bet it all on one thing though ... I'll always have a mix of hardware for mining in addition to my direct investments into various coins.
New GPUs are out. Given the price of them, I don't think they'll flood the market too quickly.