It's pretty hard to run a stable empire when economic exchange breaks down. Currency's break down as society breaks down not the other way around because currency's value is propped up by the need to pay taxes or loans in that currency. Country's that try to spend more than they take in in taxes or loans devalue there currency other than that there surprisingly stable.
Do you believe more in your government The US government is increadably well run when compared to just about every government throughout history. While it's politically useful to suggest otherwise there is a reason per capita GDP is so high and our well run government is a large part of that. Now that's not to knock bitcoins but your comparing it to something with a 99.99+% chance of being around next year and that's a tough act to follow when a single bug could kill bitcoin in a few seconds. Granted the odds are much worse 20 years out, but you get time to move currency around in most failure modes, were again bitcoins can hit zero with Zero warning or just lose 90% of it's value with a mid sized sell off.