Montparnasse is a perfect example of cities in France going "on a second thought, let's not build more skyscrapers right in the middle of cities" in the 70s. See also: Part-Dieu, Tour Bretagne, Pleyel.
It's obviously hard to pin down, but apparently it took Parisians about 20-30 years to go from hating the Eiffel Tower to loving it. Tour Montparnasse hasn't managed to achieve that in the 50 years since it was built.