They can't demand what you don't know, and in general you don't know or have any way to know who paid you, either physical identity or coin source.
Cripes man. From the beginning I have stipulated that my statement applies when there is a traceable asset traded, e.g. house, car, fiat. I wrote that cash is going away, and all fiat will be traceable (electronic).
Exactly. The car dealer is engaging in a traceable transaction. He got your ID and can identify you to the authorities. It doesn't matter what you use to pay. Cash, gold, bitcoins, Moneros or "fully anonymous" pixie dust crypto coins, its all the same at that point.
But once they get to you with a well implemented ring signature-based coin, the trail ends. There is no way for you deanonymize people who sent you coins. At best, they get some list of candidate outputs and can try to trace them back to some individual, but this too is likely to work far less well than it does for Bitcoin, perhaps even provably so (though we aren't quite there yet).
Unless you collected the sender's ID as the car dealer did with you. If we all turn into car dealers recording IDs for everyone we deal with (and presumably reporting that information or at least having a recordkeeping requirement for it), then no coin technology is going to help you.