Getting to true sustainability is increadibly difficult, let alone for the 8-10 billion people on earth. But, if you build a sustainable civilization for 2-4 billion people, it is much easier to achieve, and ypu can return a lot of land back to wilderness to allow the rest of earth's species to flourish. Its an essential component.

Ecologists talk about it in terms of I=PAT. Impact = people x affluence x technology. If you cut the number of people, everything becomes easier.