Wouldn't the whole financial system itself be vulnerable to Quantum computing.  After all, the FIAT currencies are just numbers in a computer and if quantum computers can crack any cryptography instantly doesn't that make any networked computer an open book? 

So when quantum computing is sufficiently advanced everything that is secured is crackable so I wouldn't be too worried about Cryptos as I am sure that the central banks of the world will be rushing to find a workaround which we can usurp.