The form of money evolves, there was a time when people use barter trade instead of paying in the form of money, then after some time coins replaced the barter system and as time passed those coins became heavy for those who gained more and so they introduced paper money but still coins are still used and up until now paper money and coins are still used even though new payment methods were introduced like payment through credit card, debit card and now the cryptocurrency. Even now there are still those who prefer to use paper money and coins instead of credit card or debit card, what more if it is cryptocurrency? So it really is hard to replace fiat currency because people got used to it thus making cryptocurrency just an option of payment and not a currency itself but if just everybody would accept cryptocurrency as a currency then it may replace the fiat money that we are using right now.