*there has to be someone willing to say "I'll give you X USD for Y bitcoins"

Not really. They have to be willing to say "I'll give you X thing that you value in exchange for Y bitcoins." The entire point is that the design of bitcoin makes that scenario likely to always be true precisely because it is nearly impossible to stop. As I said above, the very fact that it represents a system that is very difficult for state actors to attack ensures that it will almost certainly be used for exchanging goods and services that governments would rather not exist. Not a terribly noble future, perhaps, but certainly one that ensures bitcoins have value.