I also believing that bitcoins can never ever replace fiat currency evn if bitcoin will be the worlds currency.Because fiat is owned b government and not the same as bitcoin who is owned by rich peoples.But bitcoin can be in fiat currency line but can never ever replace any fiat
It depends on the power of the ordinary people. If everybody wants to use the bitcoin, bitcoin will replace fiat.
Even though that would happen but to see that happening is very impossible as in close to none, fiat will always be useful and bitcoins is mostly for investment as it has not get the support we need from the government.
Government support is impossible, as they will only support their own currency on which they hold total control, and they will never allow any digital currency to replace their national currency.