Good point. One way out of the counter-intuitiveness - which hasn't gone away for me, despite your explanation - is to deny that "it scales." I.e., deny that the badness of creating a just-barely-worthwhile life, into a world containing many good lives, scales with the number of just-barely-worthwhile lives. Something approaching a maximin view - the idea that in population ethics, there's a fundamental component of value that depends on how the worst-off person fares - while I wouldn't agree with it, doesn't seem so implausible. And I think it would get you many of the conclusions that you're after.