Overpopulation and lack of resources is a serious issue that doesn't receive enough attention. Not having kids isn't an option. I wouldn't even be certain switching to just adoption would be. Yes, we need high rates of adoption and I, for one, would rather adopt a child than have my own.

However, if you look at China and Japan, you see coming problems with a lack of replacement. Just not having kids leads an inverted population pyramid (link). China, after years of a one child policy, is relaxing the policy to try and offset the same trend. Last I looked, the replacement rate was 2.1. That means, to maintain a steady population people should have no more than 2 children.

The solution is tricky. China tried to mandate it, and we'll see how that works. Japan, as a modern career-driven society is naturally declining. People want to prioritize their lives and careers over children and now Japan could soon be without a labor force. I also saw something about declining fertility rates, so it could be that we'll have an actual Handmaiden's Tale situation and everything will sort itself out.

The poverty issue is tricky. It's two-fold. On one hand, you have lack of education that may result in inadequate use or misunderstanding of birth control. You could have religious reasons for not having birth control (eye roll). But another simple explanation is that areas with high fertility rates usually mean couples have more kids because...well you never know if you'll still have the first few in a year. In countries without adequate social support for elderly populations (care homes, medicare, etc.), then having more kids in insurance that you won't be living in squalor at 75 years old.

It's one of those difficult issues where no simple solution will prevail. I.e., just not having kids won't help, but adopting is certainly a step in the right direction.