You're forgetting the other very significant part of cryptography on the blockchain: the signing of transactions with wallet keys. If you can crack the public/p

You're forgetting the other very significant part of cryptography on the blockchain: the signing of transactions with wallet keys. If you can crack the public/private key system (and what bitcoin uses is not quantum-resistant), then you can sign transactions on other people's addresses. Now, this is not 100% catastrophic in principle: a break requires an address to have signed a transaction before, so in principle if you are cycling addresses like you're supposed to, you are safe, except you can't publish your transaction to the mempool anymore without risking being sniped by someone else, you need to send it to a miner you trust, which is somewhat damaging to the whole 'trustless' part of the idea. This is one reason amongst a few to be bearish on bitcoin over longer timescales, even as a 'store of value'.