Historic preservation should be limited and well defined. Landmarking a masterpiece like the Flatiron Building or the old Penn Station is sensible. Preserving a post-war glazed-brick building is absurd. But where do you draw the line between those two extremes?
My own preference is that, in a city like New York, the Landmarks Preservation Commission should have a fixed number of buildings, perhaps 5,000, that it may protect. The commission can change its chosen architectural gems, but it needs to do so slowly. It shouldn’t be able to change its rules overnight to stop construction in some previously unprotected area. If the commission wants to preserve a whole district, then let it spread its 5,000-building mandate across the area. Perhaps 5,000 buildings are too few; but without some sort of limit, any regulatory agency will constantly try to increase its scope.
The problem gets thornier in places like Paris, practically all of which is beloved worldwide. In such cases, the key is to find some sizable area, reasonably close to the city center, that can be used for ultra-dense development. Ideally, this space would be near enough to let its residents enjoy walking to the beautiful streets of the older city.