In the 1850s, St Helena's capital, Jamestown, was one of the busiest ports in the world with 1,000-1,500 ships passing through each year. East Indiamen stopping on their journey home to England, American whalers revictualling after hunting sperm whales in the Southern Ocean, and Royal Navy ships intercepting slavers making the run between West Africa and the Caribbean - and dropping the freed slaves off in St Helena to recover - all meant that Jamestown was well known in shipping circles. St Helena's private sector flourished, as a full range of services were provided to the visiting seamen and passengers.
Thirty years later, nearly all of the ships had disappeared. The island was in crisis. Large numbers of people emigrated to the Cape to find work. What happened? Several disruptive technologies hit at once. The Suez canal opened in 1869. Steam replaced sail, and quicker journey times meant that ships didn't need to stop for water. The whale population fell dramatically and gas began to replace whale oil for lighting. The slave trade ended. St Helena effectively disappeared off the map.
Lesson: while the sun is shining, by all means make hay, but build a barn and think about diversifying.