It is not really true that nothing backs up fiat currencies. They are limited to what they can print by their economies and their ability to put value to the bills they have in circulation. If they print too much the value of their currency goes down and their printing money is useless. If this was not the case, every country would just continually print money. Also governments have a track record of paying back what they owe which props up their currencies. Unfortunately cryptos have none of these factors in place. We only have to take their word for it that they will make good on the currency they have out there. However they are all overvalued right now and none of them could pay off their "valuation". This is a problem.