It is a fact that only few countries are into bitcoins most of them still doesn't know what bitcoin is. And it is an impossible thing that bitcoin will replace fiat because it will always be preferred for the transactions that are happening worldwide and countries will not gamble their government upon trusting and replacing fiat with bitcoin.
To make bitcoin replace fiat it must be teached at schools to the next generation. The old persons are addicted in old behaviors, that means fiat and not crypto currencies. On long term bitcoin can replace fiat but it will take too much time.
Luckly it will happen in decades, but there is the possibility to happen only after a century or so.
i don't think that bitcoin will replace fiat in short time because there is much thing that bitcoin and blockchain to do, beside of the price is change too often, maybe if bitcoin price is stable like gold then bitcoin can replace fiat but the sign to get in that moment is always possible as bitcoin having a good future for the economic aspect.
although bitcoin and fiat have the same function, as a means of payment and transactions, but bitcoin is still considered complicated for others, because bitcoin is not only virtual money, but people who want to know bitcoin have to understand the technology, how it works and the actual function of bitcoin itself, in contrast to money or fiat that has tangible form, it will be believed by many people, besides, bitcoin is still in a certain group, a group that knows Bitcoin, .. so, I also thought the same, for now, Bitcoin can not replace fiat in all transactions,, but perhaps the future of bitcoin can replace fiat in various ways and with the development of better bitcoin.