> And the absence of those signs would be relatively strong evidence that life, while common, isn't long-lived.

Stars are abundant, long-lived, and go through cycles of life and death.

Now look at the night sky. The chance that you eye will detect a star at any given patch of sky that is roughly the size of a star, is nearly nill. That is not bad news for those who wish to see that stars continue to exist - it is a feature of the size, vastness, and expansion of the universe. Same for life, presumably.