Bitcoin can only become an additional digital currency instead of credit cards, but cash will remain for an alternative payment option.
To be honest, I cannot agree with that. Bitcoin is a way better currency than fiat and requires less time for transactions, besides that, the fees are really small. I think that bitcoin will definitely replace fiat at some point, the only question is when it will happen. Also it shouldn't be just bitcoins, there are tons of more cryptocurrencies that have the potential on becoming even bigger than bitcoin is right now.