Bitcoins could never replace fiat currency. Why? because governments wont allow it to, just like US government wouldn't allow Saddam Hussein to denominate Iraqi oil in Euros.
Oh well that's a really true fact , if a country want they can surely adopt for the use of bitcoins in their country but yeah if any of the country decides to adopt bitcoin for foreign trade it would be a indirect call for us to invade them. I actually feel that the main use of bitcoin is for foreign trade and not adoption of it as a fiat , but until us is there none of this will be ever possible.
You'd be surprised how many countries the US has directly or indirectly invaded as a means of securing their currency dominance globally. Just within the past few years we've been seeing some larger countries the China and Russia starting to play outside the box and using their own currencies, but for what was probably the last 50-80 years the US has had next to complete control and no-one was willing to stop them.