All good, but too anthropocentric. Granted, that's what we are conditioned to use as justification for changes or systems, but if we don't ask the overall question, "What are people for?", then you just create a very efficient way of consuming the planet with human bodies.

If we don't know what humanity's goals are, then we can't say how many people there should be. If we can't say how many we need, then we aren't actually being useful to the universe: we're just avoiding natural physics. Humans evolved as a useful part of their environment. Civilization separated us from that environment so we could avoid responsibility (response ability) to it. The test of our intentionality is whether we can find a way for city-based society (civilization) to be useful to the future of the physical universe that it depends on.

If not, then we are just another failed species. If we stop chasing our tails for the benefit of a System of systems run by the mean Mean, then we can thrive and travel to the stars.