Paper 1 real-world example: Natural monopoly
If you wanted a business with a very good chance of making a big long-term profit then it may well be in water supply. Setting up a domestic water supply business means a massive initial investment in capital in the form of reservoirs, piping, sewage management, and water treatment but once it is set up you have a product that everyone needs and there are no real substitutes. This is a classic natural monopoly...