At some point improvements in computing will mean that the "crypto" in current cryptocurrencies will no longer be secure, and therefore all current currencies of this type will have to be abandonded for something else - quantum-something.
Eh, you just need a different "crypto" that's quantum safe. Yes if someone today had a computer that could crack private keys, all current crypto would be dead immediately. If crypto has some advanced warning (enough time to implement a new quantum safe private key scheme, update node software, and allow users to move funds to quantum safe addresses), then it's not existential.
