The problem with that is that the only people having less children are people who care about the environment/impact on the world/sate of the world. People who don't care have more children. This means that then there are more children raised in an environment to not-care.

Educated people are less likely to have children. The more educated women we have around the world, the fewer children they have. That's driving down the population faster than anything else. So, let's advocate for educating women instead of telling people to stop having children.

I want to have awesome, educated kids - and I don't want to have 17 of them. However, I don't want to get shit from people for propagating. If I don't have awesome kids, there's no guarantee that someone else will.