In reply to:

Then, after a while, they just "peter" out. Stop. Cease. Desist. People no longer post to them, and they are eventually archived. . . . I'm picking that it will die

Lay dormant might be nearer the truth. Look at the thread on handles, recently revived after months in abeyance. I'm sure that like any good rat, your rodent therad will hang around, waiting for some unsuspecting, diligent "stranger" to stumble across it, and revive, or even revivify, it.

p.s. The answer to your question, "what drives threads", is, of course, 42.