I argued that the US govt. can deploy its force to protect the dollar (as has done in the past even to the point of going to war) and the assurance of such a powerful guardian undergirds the trust in US dollar. Other have already answered as to how it can deploy force against Bitcoin (although not part of my argument). Governments can also act in concert. Invoking images of pedophiles or drug dealers using BTC would be more than enough to secure public support for such measures as blocking BTC port numbers, Deep Packet Inspection for BTC protocol content and ensuring that no legitimate retailer wants to be seen associating its name with the "pedophiles currency".