Actually, the areas outside of the Peripherique did not develop until the 20th century. What Baron Hausman did was slum clearing but Paris proper didn't become saturated with rich people until post WW2 era.
Fun fact about Paris vs Detroit is that the entire city of Paris could roughly within Detroit's Grand Blvd loop. Paris proper has also declined dramatically in population since the early 20th century, when it had nearly 1 million more residents than it does today...
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