probably one of the dangers of cryptocurrency is when people need it more than fiat. when it happens, it is likely the country's economic system will be disrupted, since most of the state revenues are also derived from taxes on fiat. when people prefer to use cryptocurrency, then fiat will not have a price.
You are right, I think, This is the worst situation to countries. When everything become digital, No real money. I think this is one of the most danger of cryptocurrency. Maybe we will achieve this moments, when bitcoin price increase more. More people will start using bitcoin, Maybe the 80% or more of the world populations. So real money will be ignored.