More than a billion people is possible within the next five years. The major challenge I see here is adoption. By the time a huge number of merchants adopt bitcoin, people from all over the world will be naturally compelled to use cryptocurrency for payments.
No, it is not possible, there are estimates that put the current number of bitcoin users at less than 10 million and you want that number to increase many times over in the time span of only 5 years? It is not going to happen and we need to accept that.