Well the answer to your question is YES!! bitcoin can replace cash in money transactions. But what it CAN do is not what WILL always happen. So it depends on the conditions prevailing in that very particular area or country, that decide how the status or position of bitcoin. Bitcoin has got all the traits of FIAT and can easily be used in its place. Now the payment mechanism through bitcoin has also been quite improvised and that has made it quicker to transact through digital payment mechanism as well. With the advancement of lightning network, the transaction fees have also reduced to a great extent!!