I've just figured it out. SPOILERS FOR S3...
You know how there's often one moment in a story that will just about jump up off the page/screen and say, LOOK AT ME! I'M THE KEY!
I had that moment right at the start of the series, in Day One -- the scene with Jack and Ianto on the steps, talking about what's happening to the children. Ianto says along the lines of: "I can get you alien gadgets and Earth tech, but I can't get you a child."
It was like a nova in my brain at that moment, so much so I immediately went off and vidded Ianto giving Jack a child. (Songvid: 'Shot in the Back' (TW, Jack/Ianto, PG-13) -- it's not great, but I had to try and capture that moment of THIS IS THE KEY.)
At the time, I assumed that a) I'd just read too much mpreg, and b) that scene was tapping into my own issues about kids (I have actually seen a kid die, so I was wary of reading too much of my own stuff into the CoE storyline).
Obviously both of those things are still true, but that doesn't mean Ianto's statement isn't also the point the text was making.
Do you know how most non-immortal people achieve immortality? Through the DNA they pass on to their kids.
Do you know which groups in society tend to have a more difficult time of passing on their genes? Gay people, for one, and especially gay men.
Artists, of course, have a long history of calling their art their children, and hoping to achieve immortality that way instead.
So this is what I now think: Children of Earth is not an artistic statement about the intrinsic nihilism of the human soul. It's a gay man's anguish at not being able to have kids. That's why Ianto had to die -- he couldn't give them to Jack; he said so, right there in Day One. And that's why Jack had to be punished -- he's been pregnant, that bastard, and can also have them the regular way a non-immortal man does, plus he can't die anyway.
I know there are plenty of other possible readings of the text out there. Go ahead and put them forward if you like. But it'll take a lot to convince me this isn't the driving force of the plot. If you start by telling me RTD is a happy father of three, *then* I'll be very likely to believe you.
I've x-posted some of my meta about Torchwood season 3 to torch_wood: Meta: Who is Jack Harkness?