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.
In theory it should not be particularly impossible to recognize individuals by their genetics then all kinds of interesting things become possible. Among them, designing something like a 100% mortality illness to which only a specific individual was susceptible. In that case it would be fairly straightforward to issue and execute capital punishment order without the hassle of trying to round someone up.
A slight modification would be to have a cure for the otherwise terminal illness in which case anyone who was wanted by the authorities would be pretty likely to turn himself/herself in. Should they still refuse to do so the problem will solve itself.
Of course what we'll need is a fairly complete catalog of every individual's genetic material and a reasonable mapping...I'm sure nobody is working on that.
(Oh ya...best not to be an identical twin if it can be avoided...)