Interestingly, many other nations had shelter programs during the Cold War, well aware that such things greatly enhanced the likelihood of survival. Some were wide-scale government programs, like that in Switzerland.

The "On The Beach" view of nuclear war as the Absolute End of All Things seemed to be almost exclusively an American/British idea. Certainly our Cold War adversaries, and even many of our allies, didn't see it that way.

Even the most rudimentary fallout knowledge and preparation vastly increases survival chances in an attack, even without specially-built shelters. Such knowledge is not obsolete in an age where nuclear accidents or terrorism are much more likely than nuclear attacks. The kind of non-radiation supply preparations typical of many shelters would also be useful for natural or other disasters. And as you point out, you can make a den out of it. It's actually not that hard to modify part of a basement into a decent shelter without having to dig a hole.

Opinionated rant follows:

I watched a TV movie years ago called "Testament." In the movie, a small town north of San Francisco is essentially unharmed after a nuclear war, except for the eventual fallout. I watched as the town committed suicide. Overlooking shelter options, failing to take the simplest precautions, going through their days as though nothing could be done, and killing themselves and their children out of sheer, bloody ignorance.

Most horrifying scene was when a child told her mother "Mommy, there's dust on my plate" as he ate breakfast in the sunshine near an open window. He was ignored. Later, he died.

The Conventional Wisdom was that Civil Defense in the U.S. was useless, and maybe even counterproductive because it might "fool" people into thinking a war could be survived by anybody at all. The Enlightened, who wouldn't know gamma from alpha, knew we'd all be plunged into nuclear winter and everyone on Earth would die.

If we actually had suffered an attack, such views would have been responsible for the deaths of millions.