If it were the case that the main currency of every country in the world is Bitcoin. Then how do you think the world economy would be run? Because when Bitcoin becomes the main currency, the American dollar and other countries will have Bitcoin as their main currency. The local currency of that country will be associated with it as the second currency. In this case, how would Bitcoin be used or how can it play a good role in the world economy?
I don't think that it doesn't have to go that way, that if Bitcoin becomes a global currency, every country will accept it as a legal tender. It doesn't have to make the other fiat currencies to be boycotted and make people stop using it. From what we can see, it's going to be as simple as an alternative payment method and currency, just as it is intended to be. The best comparison would be like this:
Before, the merchants and cashiers are going to ask you, cash or card?
Now, it's going to be, cash, card or Bitcoin or any other crypto?