Eventually they will become more scarce and more hence more expensive. Looking to that day, human are developing new ways of using the sun's energy that are renewable, that do not rely on fossil fuels. Falling water for one. This is one of oldest of the renewable ways that uses the energy of water lifted into clouds by the sun, after it falls as rain and flows in rivers back to the sea. Agricultural societies tapped falling water energy to grind their grain and to power early factories.
Modern societies use the same energy to create large amounts of electricity.
During the agricultural revolution humans learned to tap the energy of the wind to power sailing ships. Countries like Holland used the energy of the wind to pump sea water and thus to reclaim land from the sea. In modern times when the wind blows, the blades of newly designed windmills turn rotors in electrical generators to produce electricity. Wind farms like these are going up in California, in Denmark, in India and China and in many other advanced and developing countries on all the continents of earth. Farmers have long used solar energy to dry their crops and cure their animal meat and skins.