If it ever becomes a world reserve currency used for payments all around the world like some people believe we will have to think of something.

By the time Bitcoin reaches 21 million we will probably have at least 1/4 of that permanently lost and another 1/4 in safe accounts of lifetime hodlers which will leave only 10 million in circulation. That's not enough for over 500 million people. I said 500 because not all people in the world will use Bitcoin just like USD and EUR are estimated to be used by about 350 million people each.