I guess a good analogy would be stocks. A company sells all the shares at the IPO and then other folks continue to buy and sell without more shares being created.Yes, a company CAN issue more shares in the future which dilutes the value of the original shares which is why a finite supply in BTC is a good thing.
Fiat is the perfect example of why creating more shares/dollars/currency is a bad thing as it continues to dilute the value. A dollar in 1900 could buy a lot more than a dollar today.