Imagine what will happen if Ethereum goes POS, 2-3 million or more GPUs have to go somewhere or get sold. Interesting times ahead indeed!! 
The method will be implemented slowly. It's not gonna be like today you mine etherium and tomorrow you don't. Apart from that there're a lot of valid coins profitable to mine. And even if all miners swap to them they will pull the diffuculty up to the skies and the price of the mined coin correspondingly. Ethereum itself wouldn't have cost so much had it been not a big deal to mine some. Total network hashrate defines the price.