IMO the biggest problem with the post is that it is ignoring that natural world completely.

We can't colonize mars, not because it's far away and hard to get to (although those are problems). The real issue is that we don't really understand our own biosphere enough to build even an imitation one somewhere else. The ISS is orbits so close it's barely out of the atmosphere. It's still well protected by the Earth's magnetic field, and gets regular deliveries of food, water, spare parts, etc. Every time we've tried a closed biosphere (biodome?) on earth, it has failed.

The bigger Earth's population, the shorter the timespan we have before we can realize we screwed up somehow (i.e. overusing artificial fertilizer, emitting too much carbon, etc.) and having to urgently fix it or the whole planet is wrecked. If we had a "planet B" it wouldn't be so urgent. If we knew perfectly how the ecosphere worked, we wouldn't screw up. If we had "save points" and could just load them if we screwed up, then we could run closer to the edge and go back if we messed up. Unfortunately, this is the only planet we have, and we still don't know how it all works. Because of that, we should really run with a much lower population so that when we inevitably screw up there's a buffer to protect us while we adjust.