Monopolies can still exist in a finite currency system, but yes since more money is not created eventually things get worse quicker. As the wealthy accumulate the majority of the money, they hold more money than they can spend while the majority does not have enough money to survive. Since the banks/gov is unable to create more money the economy collapses and/or a revolution starts. In order for any economy to function the middle class has to have the most amount of money to keep the velocity of money high.