I hope I'm wrong but I believe if and when eth goes POS, gpu mining will stop being profitable for most people. Right now most gpus are hashing eth and when that huge hash power is turned to other coins, their difficulty will shoot up and profitability will plummet. I can't see any other alternative to this happening. I doubt we will ever see a bull run like this again for gpus. But it's been a wild ride 
Nope. Who will buy GPUs? Gamers will not but 10-100-1000 GPU per one man. It will be new coin, that will be mineable and gives more opportunities then ETH. As long as i've been mining there is always demand for GPUs. Whenever there is a crypto bear market and gamers think they can get "GPUs for almost free" it never happens. I've always was looking for opportunities to score great deals on GPUs during bear markets however it never happened.
Only good deals on GPUs were the ones with broken fans since most gamers don't want to deal with replacing fans. However there isn't going to be a time when you can buy a 1080Ti for like $20 because ETH all of a sudden went POS. There are lots of uses for GPUs rather than mining or gaming. They will always be a hot commodity.