8 billion people is absolutely sustainable, we could support significantly more at a modern standard of living with just the resources we use today. The problem is the way we organise how and where we live, and a parasitic owner class using and abusing vastly more resources than they could ever need.

• Education
• Opportunity
• Help those who don't want to give birth not to give birth
• Reduce the influence of religion that promotes childbirth and irresponsible family planning
• Reduce the influence of pressure to grow in every way that is likely exacerbated by capitalism

And if after all that people still want to have children?

Nature will bring our numbers to sustainable levels if we don't do it. Nature will not be so kind.

Let it try, we'll see who wins.

Nature will bring our numbers to sustainable levels if we don’t do it.

Are you proposing that we do it? How exactly does reducing the number of humans work, if we do it? Is there any word for this that isn’t “genocide”?

who do you educate? those people will be genocided.

who do you give birth control to? those people will be genocided.

there is no policy you can create and implement that will not disproportionately effect one group over another.

it's all genocide.