The hypothetical was that the US "bans" bitcoin, presumably meaning it becomes illegal for US financial institutions (or US-dependent ones, which is nearly all

The hypothetical was that the US "bans" bitcoin, presumably meaning it becomes illegal for US financial institutions (or US-dependent ones, which is nearly all of them) to convert bitcoin to dollars. Somebody else might give you dollars for bitcoin, but then it becomes their problem. As the saying goes, "you can't eat bitcoin".