after etherium all coins will collapse. no coin can absorb so much mining power. it will all depend on how many miners will be transferred to another currency. ethereum classic will absorb most hashrate as it is a similar currency and has quite good prospects. unique solution buy the coin you are mining, only this way the coins will keep their value.
Not entirely correct. GPU mining was supposed to die back in 2017 as some said but that did not come true. For the short term, it will have a significant impact on GPU mining profitability with all the hash power thrown around causing volatility. But it may ultimately settle down with a few coins, and if one blockchains has a strong/viable use case, we may have another ETH like run in our hands. Just don't go out investing in new GPUs at this time, but down shut down operations yet either.