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.
This is when various countries has accepted it has a legal tender and are willing to use it , but as it stands now most countries leaders are scared of Bitcoin and not wanting to adopt it because they believe that Bitcoin will take over their traditional currency because of its decentralization, autonomy and mainly being a digital currency that can be used without permission, even with these that the presidents are refusing, the politicians of various countries are into Bitcoin, investing as much as they want, despite that fact, some politicians are Bitcoin friendly and they are investors and consumers that has made them indirectly moving after turning their countries into a more pro-bitcoin countries and having their foundation into a Bitcoin community .
As it stands now Bitcoin is well known globally now as a currency that can be sent and received wherever you are in the universe, as long as you have a Bitcoin wallet wherever you are even if not acceptable by your countries President