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.
Bitcoin competes with card payments and banks rather than with paper money. Paper money will never disappear due to working offline but it is currently a tiny proportion of the financial system, the vast majority of money is already digital on peoples bank accounts.