It sounds easy but hard to be done. There's many countries, still doesn't recognized bitcoin.Even lots of people stay away from crypto because of volatility in crypto market. and so is the reason why big organisations still doesn’t adopted bitcoin for their payment method. Its easy to carry your assets everywhere even during war but not everyone will be interested to deal with you via bitcoin, they still need fiat for that
^ It should fiat must exist and BTC is the alternative to fiat and that is the fact, nothing else.
How can you convert to your own currency when you have BTC? In fact, BTC is decentralized and fiat is centralized they are completely irrelevant to each other, and also they are both useful to us. There is nothing to choose which one, it should be both.
Both of them have disadvantages and advantages so comparing them is worthless.
Perhaps, the creators of Bitcoin never think this will replace fiat in the future, only us who are too speculative in this matter which in fact, will never happen.
Bitcoin and the rest of altcoins will remain alternatives, that is the main goal of such creation. Nothing it says that it will replace and OP got it wrong. And even evaluating the market situation of Bitcoin, many areas where this is still not accepted, and to say this will replace fiat, which is really far from happening.