We have to look at where the main source of demand is coming from, and by looking into that, it's safe to say that Ethereum has been the most GPU driven coin in this whole space.
If we go a bit further than that, then the following things have to be taken into consideration when it comes to Ethereum;
# block subsidy reduction.
# the transitioning into a POS network.
Depending on which happens first, this will guaranteed result in less miner demand, especially with how Ethereum won't be scaling up any time soon and thus there is no reason for the price to compensate reduction in block subsidy. Ethereum's
hashrate has taken a dive and I don't think it will stop any time soon.
Either way, once Ethereum is full POS, the global demand of GPU's will sink down with at least 50% which is an advantageous event for all non crypto users of GPU's. There is no way the current number of GPU's put to work to mine Ethereum can be put to work to mine other coins profitably. They will end up on the market eventually.
That's not all, what if other GPU coins think it's good to transition into a POS network as well after Ethereum's successful switch?