I think one day cryptocurrencies maybe will not replace fiat money, but they will be accepted in a lot of stores and places all over the world. So when the resellers and people more like to accept cryptocurrencies then fiat money, i think we got it. Because fiat money will not dissapear in the next twenty years.
in fact, this has already been the case in china. although it is not directly related to crypto, but they already have an app called wechat whereby everything over there can be paid by this app. so they already considered to be cashless society already.