Yes, the question is if the thing they mine makes sense financially. If overall, people will use the POS chain better, the value of POS ETH will be much greater. So, the financial rewards, in USD, buying power, whatever, will be lower. They will also flock to other coins and they will increase difficulty, getting diminishing returns. So overall market cap of rewards decreases, the amount of hash power increases leading to much much much lower rewards.
I am willing to bet money that after ETH POS transition, graphics boards prices will crater.
> Perhaps it wouldn't happen overnight, but wouldn't ETC prices rally with more people participating in the network?
If my understanding of the crypto economics is right, no. The token price drives the mining effort, not the other way around. People buy these tokens because other people are willing to trade at that price, not because of the mining power within. Having more hash power does nothing for me as an owner of a token. The auto-scaling of the difficulty level is there to make attacks more expensive as the token value increases, it maintains the cost/benefit balance out of whack for any attacker. Only difference is slightly longer block mining time. Overall, nothing a normal user would feel.
Some people claim the other way, but I think they simply misunderstand the incentives. There was news that the Kazakstan crackdown led to the Bitcoin slump at some point, but that is, IMO, wrong, and they mistook some other events as causation. (in the same period, the stock market was also down).
GPU mining will still be a thing, but there simply will not be enough value to go around and the mining will crater until profits will be sustainable again. Extra mining gear will be sold. I maintain my prediction, and am really willing to put money where my mouth is.
I am willing to bet money that after ETH POS transition, graphics boards prices will crater.