The Eiffel Tower was supposed to be torn down after 20 years

Gustave Eiffel built his iron tower for the 1889 Paris World's Fair, and Parisian artists and intellectuals at the time called it an 'iron monstrosity' and a 'disgrace to the city'. The structure was granted a 20-year permit and was scheduled for demolition in 1909. It was saved only because its antenna proved invaluable for wireless telegraphy — particularly for intercepting German military communications during World War I. The tower's survival was accidental; its fame permanent. It remains the world's most-visited paid monument.

Source: Loyrette, Gustave Eiffel (1985)