I think it's not going to happen because cryptocurrencies has no physical form, not all the people know about this kind of technology or know about the existence of digital coin and you must have a internet connection and not all the people are afford to buy a gadget for using cryptocurrencies and paper money was used over a 100 years so it's not easy to replace.
The biggest problem when we use cryptocurrency to replace paper money is cryptocurrency can not use offline. If we transfer cryptocurrency to paper wallet, why do we not use normal paper money? This is the biggest problem make cryptocurrency can not change the best function of paper money.
Yes it would be the problem facing by people when crypto currency replace real money that we cannot use it when offline. But in this latest technology era the use of internet is too common. The crypto currency disappear the real currency by it best properties over the real money. There is no cost of paper which affected in printing of real currency. There is no involvement of banks in digital currency which has been occurred a lot of fees and taxes.