It would not threaten the dollar. In the best (worst) case it would become the new value backing of the dollar (i.e. "What could I get for this dollar?" "Well h

It would not threaten the dollar. In the best (worst) case it would become the new value backing of the dollar (i.e. "What could I get for this dollar?" "Well how many bitcoins can I buy with it?"). It could become this gradually and without any cooperation of governments or banks at all. Of course, declaring it illegal would throw a pretty big wrench in that process, even though in theory Bitcoin is not really affected by government regulation.