Most quoted, least applicable to Earth as it will be at end of century, with much arable land lost to desertification, inundation, saltation, sprawl, and aquifer depletion. Sustainability is defined as over the long haul, with all renewable resources.
Reduced population doesn't actually harm your Libertarian capitalist vision, so why the defensive giggles? As many economists have pointed out, you can have more productivity per person, and it's far more sustainable to have greater individual wealth, when population is stabilized at the lower figure.