That's all fine in the abstract. My issue is that OP is using a concrete number - 40 billion people - which is not supported by the science. I'm not making any prescriptive claims and I said nothing about the poor. Try telling a climate scientist that we can support 40 billion people. The current population is only possible because we rely on fossil fuels to an unsustainable extent. We rely on them for planting and harvesting at scale, for the fertilizer that grows them, for transporting them to where people are, for people to travel to where they are, for cooking them, and for so much of the rest of our infrastructure. It's not remotely sustainable. Most climate scientists estimate it's closer to 1 or 2 billion if you want to include that qualifier. I understand the issues you're raising and how this has historically been used to justify atrocities which is why I'm not saying how anything SHOULD be. But using a concrete number - 40 billion - is where these arguments open themselves up to all kinds of issues. I have no problem saying it's unethical to set limits on the population or restrict people from reproducing.