Imagine what would happen if one person held literally all the money. It may be easier to imagine a 100-person community rather than the whole world.
One thing that would have to happen is everyone else would develop their own money/currency, which would turn mean that the one person with all the money is no longer the one person with all the money.
A thing quite likely to happen is that everyone would decide that whatever the money is that the one person has, everybody would decide it is not money.
It isn't entirely dissimilar from me claiming I have all the Jerfoleans on the world, and what the world is really doing about that currency right now. They're worth a zero so zero-y that even an infinite number of them is still worth a flat zero. Someone who has "all the money" might find themselves in a similar boat.