Affirm CEO Max Levchin on why the card payment interface remains unbeaten, and why better ideas fail without reaching critical mass: "We're forever in search of the great way to pay biometrically, and it manifests itself more in movies where people's thumbs and eyeballs get removed for authentication purposes." "And yet we're still paying with chips. By the way, the card payment interface is the singular best user interface ever created." "Even before PayPal there was this wand... where you…