I see no reason to believe the second scenario is anywhere close to occurring. The gains we've seen in life expectancy over the past century or so have been due

I see no reason to believe the second scenario is anywhere close to occurring. The gains we've seen in life expectancy over the past century or so have been due to things like reducing child mortality and better treatments for disease so less people die younger. The maximum life expectancy of a human is about the same now as it was several thousand years ago.