your assuming that all other currencies are rejected worldwide and bitcoin emerges the dominant and sole means of currency with that.... which i really dont think is that likely as long as there is real and solid GOLD SILVER AND OTHER PRECIOUS METALS available on the planet.
This is true. But it will be something like this: "Sir, please scan your right forearm here so we can buy that gold from you."
It is not inconceivable that every baby will have its DNA stored on a decentralised protocol for id purposes. The justifications are; the protocol is decentralised so no one controls it, it will be pseudonymous or even private, it will ensure that the person transacting is the right person.
So for a transaction to occur, you would need your private key plus your dna signature.