Unless every single coin major coin makes the switch to PoS or other non PoW consensus mechanisms, I don't think mining will be going anywhere any time soon.
~ 17 280 eth mined daily, ~ 15 Million dollars daily. I think we would feel that dent.
Right now it's 210 Th/s on the eth network, and you need 34 000 rx polaris for 1 Th. Those gpus will search for a home.
How many gpus are mining shitcoins, that would be nice to know.