Everything will be fine with the miners, they will go to mine something else, or they will sell video cards, I think they have already recouped the costs, I expect scalability from ETH 2.0 in the first place, and POW or POS does not matter.POS for Ethereum is better but that won't be the reason for miners to sell off their mining rigs, though Eth is one of the most profitable coin to mine since day one but we have other coins using same Algorithm (ethash) with Ethereum e.g Ethereum Classic