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?
There are no such things as a "main currency" and a "secondary currency" in one jurisdiction. There are national currencies and global currencies(US dollar, Euro, etc.). If BTC suddenly becomes a global reserve asset(which is very unlikely to happen) most central banks would simply buy BTC and HODL it in their currency reserves(which would support the value of the national fiat currencies). Bitcoin would play the same role gold did during the gold standard.
Bitcoin cannot become the one and only global currency, because 1.No major central bank would allow this to happen. 2. The transaction fees on the BTC blockchain would hit the Moon. The BTC blockchain cannot handle billions of transactions per day.