Why Was This Manhattan Mansion Demolished? (Brokaw Mansion) 

"Landmark" is legal term, and a relatively new one in 1964. If the Brokaw mansions had been "landmarks," they wouldn't have been legally able to be torn down. I can certainly understand the sentiments behind not wanting to tear down old mansions or old buildings, in general, but tastes change, as do economics and attitudes about ostentation. Though you're entitled to your opinion that the apartment building on the corner of 79th and Fifth is "stunningly ugly," I believe it's the best of the several modern post war apartment buildings on Fifth Avenue. I grew up in the neighborhood, and I always admired the airy set back and driveway and all the glass allowing a view of the lobby. It's the height of 60s-70s residential modernism. Fortunately, right across the street, from the corner of 78th and Fifth and all down 79th Street to the corner of Madison is a perfectly preserved two block row of contiguous mansions. Two are presently for sale. You can check them out on Zillow. Ronald Bloomberg lives right behind the property in question in two relatively low key townhouses on the north side of 79th. The super wealthy don't often want to live in Gilded Age like ostentation, at least in New York. Though there are many blocks of townhouses, there are far more apartment buildings housing the super wealthy. There's a bit more anonymity and security comes with ownership stake in the cooperative.

They were in the way of progress.

@@garryferrington811 I don't agree with your comment. It was greed on behalf of the city authorities, which changed the zoning law to allow high rise buildings in the area to increase their tax base, and greed on behalf of developers to make more money.