i do not thing that it can be happen in the near future, but hope that after decades we will be able to replace fiat and start using bitcoin in all kind of activities.
If we imagine such a situation without cash, then remembered the fantastic films where, instead of cash or credit cards there. But still there is something in his arms, not virtual. Let there be no paper, but still something will be different.
So we can just back in the ancient times and keep on the trading with other goods like gold and other precious metal again and that is barter.
But I really don't think something different will make the people not use paper money. Because this is supported by the government.
Unless the government will support bitcoin then they can just implement it.