[throwway120385]

There's an unfounded assumption in what you're saying: people make mistakes. In the current system without non-repudiation, we can repudiate our mistakes and get our money back. In your crypto-based system, if someone tricks me into divulging my keypair and impersonates me, I can't repudiate that transaction so it's gone forever. And so you're perpetually one mistake away from losing everything which was what the consumer protection laws are protecting you from in the current system. They protect you from losing everything for one mistake.