PARIS — When I. M. Pei’s glass pyramid opened at the Louvre more than 20 years ago, many argued that this 70-foot-tall structure had destroyed the classical beauty of one of the world’s great museums. But today, as crowds wait on long lines outside the pyramid, which serves as the Louvre’s main entrance, what once seemed audacious has become as accepted a part of the city’s visual landscape as the Eiffel Tower or the Arc de Triomphe. Now the museum is again risking the public’s wrath as it introduces the most radical architectural intervention since the pyramid in 1989. Designed...