[UkrainianJew]

>I can't think of a way to do things like proving that a particular debit happened.

Prove to whom? Banks have multiple layers of redundancy (including internal logs, paper/PDF statements, etc). So in case of any software error or a single-employee malice, it will be just fine.

>Part of the reason that bank errors are so inexpensive to fix is because transfers take 2-5 days.

That's a feature, not a bug. It gives you time to stop a transaction initiated by someone who stole your identity. With blockchain, you lose keys - you lose coins.

[mgraczyk]

It doesn't have to be that way though. There's nothing inherent about the timescale of 2-5 days that prevents the same level of security and reversibility with shorter transactions. The problem is the lack of coherence, lack of information, and lack of fast technology. It's easy to build a reversible version of BTC with programmable timeouts for reversal, and the timeouts can then be tuned based on transaction size, risk tolerance of the sender, and willingness of a third party to underwrite the transaction risk.

Credit cards basically do this. Part of the reason they are able to is because they gather more data and rely on more sophisticated technology than the banking system. With blockchains, you have another small tool to improve the situation.