New York City’s landmarks preservation law turns 50 on Sunday. Any who doubt its continuing significance should head to the West Side of Manhattan, to the building it came too late to save. Pennsylvania Station in 2015 is a monument to civic suffocation, a basement of low, dust-blackened ceilings, confusing corridors, beer-and-popcorn dealers, yowling buskers and trudging commuters.

Thousands of buildings, from brownstones to modernist skyscrapers, irreplaceable historic interiors and entire neighborhoods from Staten Island to the Bronx enjoy the law’s protection. In Manhattan, 27 percent of buildings have landmark status, though the percentage is far lower in the other boroughs.

The administration of Mayor Bill de Blasio, while on a mission to make the city denser and taller for affordable housing, says it is committed to finding and saving the architectural legacy citywide. It points to its recent designations, including: a children’s library in Brownsville, Tudor Revival townhouses in Flatbush and early 20th-century working-class housing in Ridgewood. 

Despite these successes, the law is not being used aggressively enough, has never met its potential, and has let too many precious buildings languish or be leveled while saving architectural mediocrities. The Landmarks Preservation Commission, with one of the tiniest budgets and staffs of any city agency, has had a bush-league reputation. Mr. de Blasio and his landmarks chairwoman, Meenakshi Srinivasan, have pledged to revitalize it, but have not yet shown how they will reconcile the commission’s mission with the administration’s broader goals of enhancing equality and diversity in the flesh-and-blood city.