The Lindy Effect posits that the lifespan of an idea is about 2x as long as it is old. If the story of Beowulf has been told for about 1000 years, we can expect it to be told for 1000 more.
Unlike mortals and machines, ideas don't wear out with use. The more an idea is used, the likelier it is to stick around. What applies to ideas also applies to things powered by ideas — technologies, companies, cultures, religions. The general notion is that things which stand the test of time have properties which will allow them to continue to stand the test of time.
There is a kind of tautology here — by this rule, all ideas should be immortal. So what kills an idea? Ideas are like lobsters. They never die naturally. Eventually they get done in by some environmental factor.