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?
It can't and people who think it can
replace every other currency are badly misinformed. The bitcoin network could barely cope with all the transactions that took place daily in a small country with a few million people. It's not even that efficient when it comes to transaction fees and there is too much volatility in the fees to be reliable - I don't want to pay 1% transaction fees one day and 5% the next, when banks can already offer more consistency. The best to hope for is a place and it's probably better if it's not officially sponsored, where it can serve larger transactions of say $500+. There are niches which it can be very successful in, like cross border trade - currency exchanges combined with bank fees can be brought down.