Relief is on the horizon. Ethereum should switch to proof of stake in June 2022 and you are about to see an unholy torrent of used GPUs hit the market. I would expect you can pick up any you like for peanuts then.
Ethereum PoW won't immediately disappear, and I'm sure bitcoin folk will be all too happy to grab those extra GPUs
GPU mining Bitcoin hasn't been reasonable for many years unless you have very cheap energy and no access to ASIC miners.
I have been hearing "Ethereum is switching to proof of stake in a few months" for at least 6 years now. I don't think it's going to ever happen.
No, miners will just move to another valuable PoW shitcoin and/or fork ETH to stay on PoW.