I don`t think Fiat Currency will get crashed because the Government are supporting Fiat Currency, so as long as Government supports Fiat currency there will be no reason for fiat to get crashed. But if the Government start transferring their support on cryptos, probably fiat money will get crashed and not be used anymore.
When the time that the people realizes that they are just losing their real value of money overtime then the people will not use fiat anymore and they will just use the cryptocurrencies and that is when the fiat currencies will crash and even though the governments are supporting it, then it is not enough for the fiat currencies to stay.