Of course I understand that if everybody use only sound money, fractional reserve could not exist. But my point is that even if bitcoin becomes the main currency of the world, it might be the reserve currency, just like gold has been. So more precisely, what would prevent bitcoin to become a reserve currency ? If at some point it is hard to get, because it is limited, most people may want to use alternative currencies, and we would be back to the same cycle again. I do not see anything preventing it from happening.