To be honest, when it happens, there will be no FIAT money anymore. The government currency system will totally collapse and there will be a revolution of Cryptocurrency which is leaded by Bitcoin users. There will be no tax in the world and it will be much better when there is only one currency in the world: Bitcoin
That will never happen fiat will always show its presence even in future and till the time Government will have their control on a country fiat will always exist as Government will never allow any digital currency to replace their national currency.
Yes, fiat will not be rendered worthless as long as we have governments that collect taxes and print money at the very least. No government would allow bitcoin to become a full-scale replacement to their national currencies. To that end, some of them take a somewhat short-sighted approach and go as far putting a lid on its use within their jurisdictions.