I think your possible scenarios -- very rare life, energy trap or great winnower / cliff -- actually are all pretty positive in terms of Drake - to me the major

I think your possible scenarios -- very rare life, energy trap or great winnower / cliff -- actually are all pretty positive in terms of Drake - to me the major point the equations make is just this: if you see any aliens at all, it is likely that you are late to the game of getting extrasolar, and due to the timescales involved, it is most likely that you are very very late to the game. That said, if, say in a billion years, we survive as a race, and still haven't met anyone, then some of those drake choices are very different than we imagine them -- and we must have somehow gotten really lucky, or there is something else very strange going on. Regardless, it would likely be pretty bad news to a single-planet lifeform to meet us at that point, or at least tectonic, depending on how altruistic society as a whole would be at the time of the meeting.