Bitcoin has one problem in that it requires power and Internet to function. In a post-Dollar scenario lets say, there is a good chance the grid goes down. If US dollars are suddenly worthless, all businesses would stop working, and commerce would shut down. Currency then would be whatever has barter value.

Digital currencies are not holding nearly enough of the economy as a failsafe against major currency collapses (globally), and the things that Bitcoin requires may not be operational again for a while.

Plus if there is no more Dollar exchange, a Bitcoin will only be worth what a Bitcoin can buy

Though I hope that because of Bitcoins open source  nature and ease of adoption using existing technology, that it may surprise us with how fast we could move to a new economic system. Though not without some amount of pain either way.

Hold Bitcoin, maybe some precious metals, and also stock up your pantry and make sure you have emergency supplies for a prolonged blackout. Prepare for all scenarios, as it is impossible to predict what happens next.