US debt cannot increase indefinitely without consequence, and in a time of desperate need (such as the 2008 bailouts) it has to print lots of money and hence cause inflation. If the US dollar suffered 10% inflation 2 years in a row, it could originate some drastic changes, as dollar holders might want to get rid of the coin. Figure china selling out its entire dollar reservers in one year, US goes to the tank.

Im not being alarmist, its just a possibility. Comparing Bitcoin to USD makes no sense at this stage in many ways, starting with volume.