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.