I think that all currencies, whether cryptocurrency or not will eventually become defunct.
In future we will have technology that renders things like copyrights, patents and practically everything else useless. People will have 3D printers that can produce materials on an atomic scale, which will thereby allow everybody to print whatever they want, whether that be food, a new book, a new house or whatever.
In a future where nothing is scarce, money has no value.
The absence of a deficit does not mean stability. You will always watch what your neighbor has behind the fence. Cryptocurrency is now a good way to overcome the inflation that can be provoked by Fiat money.