When governments can set national currency as legal tender, they control what flows in economic veins. They can advantage from it by reducing the currency value by printing. While they keep it low, so people can gain interest calculated in consuming power everything can work, but now, when most countries print so much that savings not paying interest, the believe in currency value can be undermined. That will put pressure on inflation (in novadays definition as money value which is supply x circulation) while in crisis people tend to spend less, it can work until the moment people see that money lose a lot of value, then they lost belief in currency, start to spend as fast as they get money, and inflation will skyrocket. The days of national currencies are counted, as humans we need better alternative, and one of the options are cryptocurrencies.