One of the major uses for fossil fuel energy is to add complexity to the system. For example, roads, electricity transmission lines, and long-distance trade are forms of complexity that can be added to the economy using fossil fuels.

When energy per capita falls, it becomes increasingly difficult to maintain the complexity that has been put in place. It becomes too expensive to properly maintain roads, electrical services become increasingly intermittent, and trade is reduced. Long waits for replacement parts become common. These little problems build on one another to become bigger problems. Eventually, major parts of the world’s economy start failing completely.

Our Finite World, Gail Tverberg. 

https://ourfiniteworld.com/2023/03/05/when-the-economy-gets-squeezed-by-too-little-energy/