The mining of bitcoin and other cryptocurrencies are controlled by an event called halvings. Because of halvings, only less coins can be mined over time. So, the supply remains fixed.
For example, consider there are 10 million units of bitcoin in the market. Assume that 16 bitcoin units are mined every 10 minutes. It means 840,960 coins are mined every year. Once every two years, an event called block reward halving takes place
Because of this, only 8 bitcoins be mined every 10 minutes. After the next halving, only 4 coins can be mined every 10 minutes. When new coins are needed, changing the protocol is not impossible. At some point, mining becomes impossible, and supply becomes fixed. It is not possible to guarantee if a coin be worth more after the halving.
Once new coins cannot be mined, bitcoin miners can only earn through transaction fees.