One of the terms is literally “L = the length of time for which such civilizations release detectable signals into space”. Which is to say that no alien civilization is expected to last forever and maybe not even for that long (barring any mitigating factors of course).
This is actually a part of the calculus that goes into figuring out “where the hell are all the interplanetary civilizations at?”
https://en.wikipedia.org/wiki/Drake_equation
One of the terms is literally “L = the length of time for which such civilizations release detectable signals into space”. Which is to say that no alien civilization is expected to last forever and maybe not even for that long (barring any mitigating factors of course).