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?
The economy will be in shambles, many funds will not be accounted for, and the government will not be able to enforce so many seizures. Bitcoin wasn't wired for the economy but to complement the disparity in financial flows, give more privacy and to make people have the true rights over their money.
Because when Bitcoin becomes the main currency, the American dollar and other countries will have Bitcoin as their main currency.
This will not happen. The US and many other countries will not allow what they can't control to be their national currency.