Today's kids are going to pay for your healthcare and pension when you'll be old. On average, 50% of an individual's medical spending is spent during the last 6 months of its life. Add around 30-40 years of retirement income, it's an enormous burden for the society.

Even if you're (very) rich and saved for it, the fact that less and less young people are available to create wealth means that the economy becomes inflationary, or has to use immigrants, depleting the other poor countries of their skilled workforce.

Unfortunately, a volontary "child-free" or rather childless, it's not a disease, thanks, is a kind of freeloading. Politically it's very destructive, as people without children don't want to fund infrastructure or services for them, aggravating the problem.

If you want an example, Italy is a great example of a dying society due to the lack of reproduction and the State' disinterest for it.