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.

Only the dented brasswork on some Long Island Rail Road stairways and some old photos in the Amtrak waiting area point to the half-forgotten memory of something far better that used to occupy the space.

The old Penn Station’s destruction, unthinkable until it happened in 1963, galvanized public support for a law to slow the city’s blistering pace of architectural erasure. But even that crime wasn’t enough; it took the demolition in 1965 of the Brokaw houses, a set of grand old mansions on Fifth Avenue at 79th Street, to propel the bill off Mayor Robert Wagner’s desk and into law. It created a preservation commission with teeth, to guard the city’s memory.

Half a century later, how are those teeth holding up? The answers are contradictory.

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.