In addition to block rewards, which is how miners receive bitcoins in exchange for solving the computational equations, users pay gas fees. These are costs to have your transaction verified ahead of other transactions. With the full 21 million coins in place, there will be lots of users, and lots of demand for transaction verification, and hence, lots of gas fees (both in number and price). Theoretically, the gas fees will be sufficient compensation to keep the miners motivated.
Rick Dwyer, Chief Development Officer, Strategy Marketplace | Ric responded, Aug 13, 2021