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?
Firstly Bitcoin wasn't created to replace traditional currencies it was created as an alternative currency, something different from what we're used to. I don't think that traditional currencies can be eliminated because they're the identities of countries, except if the whole world becomes one global economy. That is where Bitcoin can be considered as a unified global currency but I doubt that something like that can happen. Again Bitcoin wasn't created to make any economy better, it is a store of value for anybody, institutions or government that holds it. It's a decentralized digital currency for those that enjoys their privacy, those that are avoiding fiat inflation and those that wants to make profit because it is a valued asset.