*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.
If people are willing to trade goods for it, then someone will be willing to trade dollars for it (or yen, euros, or any established currency).
For a reason that I am unaware of, you seem to think that just because something can be traded it will be traded. That's not true. People tend to only trade things that are readily exchangeable in the larger economy - things that have value.
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.