As I said, there are plenty of profitable coins for GPU mining besides Ethereum [0]. Ethereum moving to PoS will not move the needle, people will just move on to something else.
Sure, some of it will move to other currencies, but my guess would be it won't be anywhere near 100% as you're suggesting.
Secondly, I tend to think there'll be a mass extinction event, where most useless projects will eventually die off. You can mine all you want, but if there's no real utility in yet another PoW project (and there isn't), you won't be able to sell the coins at a price that justifies running the operation.