>It means that civilizations should exist that are millions of years more technologically advanced than us; and should be leaving telltale signatures across the sky that we'd likely have detected by now.

Or they've reached their technological plateau millions of years ago. Like we did 50 years ago.

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

We know for a fact that life have existed on Earth for >2 billion years.