We do not know how to stop population decline
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The few people that are having kids will inherit the earth, but it's going to be a rough road.
Joking aside (and I know I'm the only one joking), I think the outcome is not far off from what youre suggesting. When a government has a choice between stagnating to extinction and culling their unproductive subjects, well, governments do what they have to to ensure continuity.
The world is going to be a much less pleasant place in the future if this problem isn't solved.
1) People can't afford kids.
That does't make sense. The world is richer now than ever before."
The WORLD is "richer" than ever before. People aren't.
But even if you do that, you'll have an issue in a generation or so when those childless people are old. They're retiring, they're drawing from pensions and social security programs, they're using the hell out of the medical services, and someone has to pay for that. They don't have kids to take care of them, so all that capital that went towards the children gets siphoned from them in their working years. Now they're in a situation where they can't afford kids, and that trick can't be done anymore. It won't actually get population growth up over a few generations.
Right?
Turn it into an even more obvious pyramid scheme.
</sarcasm>