Exactly and in fact some crypto currencies claim to be working on making quantum computing resistant crypto currency. So if someone made a claim like envisioned by GP and could prove it, certain crypto currencies might do just the opposite of going to zero.

Yup. Quantum Jubilee is coming. But interestingly, even just the credible threat of Quantum Jubilee could have devastating effects on Bitcoin classic. Banks might be better prepared to switch over to quantum-resistant encryption since they're already centralized.

Banks are still stuck on 70's infrastructure and hiring 70 year old programmers with 500k+/year salaries to write COBOL because they're afraid to rebuild it.

I'm not sure I'd bet on them having a quicker response than an open-source project with millions of interested parties.

I think modern society going down is a bit of a stretch here. There are quantum-resistant cryptographic algorithms that would be safe even against quantum computers.

However, EVERYTHING has to switch over before it happens. Everyone has to change all their passwords and security questions and private keys.

Additionally, anything sent over the Internet via ANY of the standardized public key encryption algorithms can be skimmed and stored right now and decrypted once quantum computers are available. Social security numbers, bank account numbers, credit card numbers, security question answers, tax returns, trade secrets, personal details, Signal messages between elected officials and/or dissidents, informants, etc... All in clear text to anyone with forethought to start storing it right now.

Changing to a quantum resistant algorithm doesn't change the past; it doesn't change data that has already been archived.

That's why Quantum Jubilee is going to be such a big deal. Not only do you have the usual slowness in deploying new encryption standards (thus likely exposing industrial equipment, etc), but everything being sent right now is already at risk.

And we don't know when it'll happen. Could be 5 years from now, could be 15. Could have already happened in secret (unlikely, but still possible).