Basic supply and demand theory. When supply goes up, price falls. Since Bitcoin's supply does not go up after a certain point, all other things being equal it will preserve purchasing power vs the USD.
but why would demand remain constant? In Bitcoin, a buyer & seller are needed in order to exchange. The idea of demand remaining constant only really holds true in a scenario where Bitcoin is the 1 world currency, and even then there are fluctuations in population and productivity.
I get that you're saying all other things being equal but that doesn't really float in a real-world simulator.
I get that you're saying all other things being equal but that doesn't really float in a real-world simulator.