/r/childfree is still a useful place for people to gain some perspective - that there are actually people who never want kids and have mostly good, personal reasons for this. Not everyone cares about the fact that other countries will likely dominate in the future. Hey, good for them.
For individuals who weigh the risks and rewards in our society it is easy to make the judgement having kids does not make sense. A lot of things we do in modern society is that individuals value their own welfare more than the welfare of the group, and are unwilling to sacrifice in exchange for the group's future prosperity. I refuse to be a disposable slave. I value my time and my life, and I am worth more long term by not taking what seems to me as stupid risks.
You make an important point, which is that the value of kids is a positive externality that is not easily captured for yourself. And that's certainly true with regards to kids: you spend hundreds of thousands of dollars raising them, but it's their employers and society at large that capture their millions of dollars in productive labor. If you care more about yourself than your tribe (and there's nothing wrong with that), it does indeed make sense not to have kids.
My objection, again, isn't to people who choose not to have kids. My objection is to the people (and I think they make up the bulk of /r/childfree) who rely on specious arguments to justify their decision, and act like they're doing society a big favor in not having kids so they can focus on their music career.
For individuals who weigh the risks and rewards in our society it is easy to make the judgement having kids does not make sense. A lot of things we do in modern society is that individuals value their own welfare more than the welfare of the group, and are unwilling to sacrifice in exchange for the group's future prosperity. I refuse to be a disposable slave. I value my time and my life, and I am worth more long term by not taking what seems to me as stupid risks.