1. If they can crack the cryptocurrencies then it means that they can easily crack crypto of online banking systems as well, entire world would crash regardless of whichever asset you hold.
2. There are already quantum resistant crypto algorithms, crypto currencies can easily switch to them once it is clear that current crypto algorithms can be cracked. It would mean that people who did transactions between the detection period would probably lose their money but it wouldn’t be the end of crypto currencies.
Bank account are not protected by cryptography, they're protected by law. What you want is bitcoin to have the same, not say that since a banks' https website could be hacked, let's do nothing about the main weakness of crypto.
But anyway the point is moot: it's certain developers would upgrade the network with time as new events unfold.
They're very different because the blockchain is public. Getting access to a bank's database touches a lot more systems and algorithms. Even something less interesting like intercepting web traffic to (or between) banks isn't interesting because you need physical access to the medium and you'd need a hole in TLS. Like Bitcoin, TLS uses SHA-256, but breaking it probably doesn't give you access to any plaintext, and you could force a downgrade to TLS 1.2 to use MD5-SHA-1. This is only per-connection; the Bitcoin ledger is set in stone.
The sudden, catastrophic collapse of all online commerce would probably be a bigger problem, as would the immediate halting of electronic interbank communication.
There was even a time, believe it or not, where electricity didn't exist and people/society still kinda worked...
When classical crypto is reverse (when, not if), then we'll simply have all our past secrets revealed. What you want is gradual increase in complexity so that the decyphered past only impact dead people.
Crypto currency will ofc do that too, don't forget bitcoin can change everything, even its 21M limit, as long as the majority of the network follows.
https://en.bitcoin.it/wiki/Quantum_computing_and_Bitcoin#Mit...