Your words are not beyond the reach of reason to suggest. Fiat currencies rule. When we talk about price of any crypto we talk about fiat (about dollars). All exchanges shows price like that BTC or alt costs "X" dollars and buy crypto by selling dollars. If we won't have dollars how can we understand the price of crypto? Crypto is inseparably associated with fiat. Think they can't be divided.
Yeah man! You're absolutely right! If there is no government backed currency, there will be total chaos and humanity will revert to a barter economy!
This is not about fiat currency disappearing. This assumption is generally absurd, since under no circumstances will states give up their national money. Here we are talking about the disappearance of paper money. It seems to me that this is also far from reality, because in many cases it is most convenient and effortless to pay with paper money. It must always be remembered that various technical means are always needed to use cashless payments. However, not everyone lives in megacities, where they can be at every turn. For many localities and certain categories of people, this will be simply impossible.
In addition, I would generally not want paper money to disappear completely and we would be completely under financial control from the side of governments, and this should be remembered by hotheads who advocate abandoning cash.