The only way fiat is going to die is when the population will lose faith in it and the government will fail. Even hyperinflation like the one currently happening in Venezuela is not enough to kill fiat, many other countries have had it at some point of their history, and they've usually solved it by rebooting their currency. Too many people are dependent on it - people employed by the government, people who pay taxes and so on. I don't believe in Bitcoin revolution, Bitcoin will just keep coexisting with fiat and will have a relatively small userbase consisting of people who value its properties.
People don't lose faith in fiat, because the government uses its muscle to ensure that they continue using it. Long before Bitcoin, there have been ways for people to stop using fiat - they could have used gold instead. But even in countries with hyperinflation, the government machinery has ensured that fiat continues to be used.
It depends on the country though. In a strong economy like the US you're right that it would take a huge shift for people to lose faith in fiat but in struggling economies it already has happened. I've been to countries that you go into a public market and the vendors don't even take the local currency anymore they take USD. In those types of places people are already losing faith in fiat and will come to embrace cryptocurrency.