> Some rules, though, are social. For example, why does everyone try to extend the longest branch in the block chain? Sure, the protocol says it's the rule, but why should that mean anything? People don't follow rules because they want to. They follow rules because it's in their enlightened self interest to do so.

You're taking a fundamentally confused position here. Consider: You have UberCoin— it is whatever spherical cow cryptocoin. EvilPal(Inc) then forks UberCoin and replaces its insides with a frontend to their EvilPal website (which then also copies in all the UberCoin ledger). By reduction, all digital currency systems are "social rules". It's not a useful description.

The longest chain rule is a rule that comes about for creating consensus in the system. If you change it in yours you will eventually break off into a private universe. The behavior actually does work by construction— but like my EvilPal example, that doesn't stop a stupid user from changing their own software to break off into another universe.

If you would like a longer formal analysis of the consensus mechanism in Bitcoin please see: https://socrates1024.s3.amazonaws.com/consensus.pdf