Absolutely untrue. I know that if I purchase something for a dollar, it will not be 1.50 or 2.00 when I look tomorrow due to wild currency fluctuations. Good currencies are backed by the full faith and credit of their respective institutions. If theoretically the US government exploded tomorrow then obviously there would be issues but if the US goes down kicking and screaming there's a lot more problems with the world. Cryptocurrencies are not the solution. They are a solution seeking a problem.