I have never heard of one of these lasting for forty years. I would wager that soon, if not already, some of the mourners' parents were not even born when the event occurred. I think you are right in that mourning and participation in the "cult" are clearly markers of belonging to the "club," and, for many, not really expressions of "mourning" in any narrow sense.

I am mostly familiar with ephemeral memorials in the context of large, public tragedies such as the death of Diana, school shootings or public murders: sidewalks become covered with flowers and photos and notes for a few weeks, and then fade away. There are also ephemeral additions to permanent memorials, not just candles and flowers, of course, but notes, artwork and even toys.

I recall how some of these sites become battlegrounds. In the United States a number of years ago, atheist groups were removing improvised crosses along roadsides which marked the sites of fatal accidents. Those who placed them were building memorials to dead loved ones. The atheists were fighting a separation of church and state battle, however. They objected to "religious symbols" being displayed on public land. The law was on the side of the atheists - it is illegal to place unauthorized signs and markers on highways. Here in Germany, these crosses tend to be left unmolested.