I'm sorry but you're getting it wrong, the way I see it, Bitcoin is not really limited like people think. Every single day there are lots of people buying and selling Bitcoin and it's never going to stop. And when the demand for it gets high the price will as well be high, which means that it reduces the amount that people are able to purchase with a particular amount. If you can buy 1 btc with $1 and there happens to be a high demand, then 1 btc will start selling at $2 and that means that you will no longer be able to get up to 1 btc with that amount and will have to opt for 0.5 btc. You see?
Apparently, it meant that 21 million bitcoins will be mined, but over time, in fact, their number will decrease due to natural decline. Already today, more than six million bitcoins from the extracted 18 million have been irretrievably lost. Over time, the number of lost bitcoins will increase. Therefore, over time, the number of working bitcoins will decrease and the time will come when there will be very few of them.
Of course, given the fact that Bitcoin is planned to be mined before 2140, these will not be our problems.