The whole world population must not own Bitcoin at the same time just as you don't the whole world owning dollar. It can always be passed on from one person to another in form of exchange for goods or services.
I get your dollar analogy but like Fiat currency(an umbrella under which dollar falls), the idea and vision behind cryptocurrency(under which falls bitcoin) is the creation of a global digital currency that can be used for trade in any jurisdiction. At least that's what bitcoin enthusiasts are fighting for, that each nation of the world will legalize the use of bitcoin for commercial trade and transactions.