Just to cite one of of your statements, the oceans have been decimated with countless species extinct and many more on their way there due to overfishing and habitat destruction. The oceans are also filled with industrial chemicals, plastics and have numerous massive "dead zones" due to nitrogen run off from the huge factory farms we run to sustain our massively overpopulated planet.
The suggestion that the earth isn't overpopulated, let alone the idea that the earth could support "hundreds of billions of people" is beyond absurd. We are losing biodiversity among animals, insects, fish and every other form of non human life due directly to massive overpopulation. Saying this ignores the decimation of our ecosystem and the food web that has left our planet teetering on the brink.