As a final note, Bitcoin is hard to come by, and if 21 million people buy one each, they will never be able to buy it again.
When the total supply of 21 million bitcoins has been exhausted, of course this will become a scarcity in the market so that it will make the price of bitcoin more valuable because the demand in the market will increase while the supply is less, this is certainly good for bitcoin holders because they will get big profits when selling it.