You made many interesting points here, some of which I wasn't aware of. But I think I disagree with the gist of this post.

For example, you mainly argue that overpopulation is harmless by taking three approaches. One approach is to say that the Earth isn't a closed system.

I want to be practical about this: space colonization is sci fi at this moment. And we aren't doing much about solar energy.

Two, you say people, in order to grow, need just food and water.

True, but, again, let's be practical: you think the rich are just going to equally distribute all their wealth? I don't see that happening now, so I've no reason to think it will happen in the future. Death and war are more likely, I think.

Your third approach is to say that people are in fact having less children. I agree with this, and it's why I think a true appreciation of the issue of overpopulation involves the realization that it's really a two-pronged problem: on the ONE hand you have overpopulation, and on the OTHER you have the bankrupting of the entire economy if the opposite of overpopulation ensues, i.e. if the average population keeps getting older because of better healthcare coupled with lowering fertility. There's no way (according to economists) the young population can sustain the increasingly older population.

Well, there IS a way, and it's called defeating aging by means of scientific immortality, but that's another topic altogether! You mention this in your closing paragraph, even mention the fact that the world is getting older and the natural follow-up to that is the transhumanistic movement, but I don't think the reader who doesn't already know about this stuff appreciates how that relates to the rest of your argument in the post.

I also agree with your meliorist stance at the end of the post: human ingenuity. I too am hopeful, and even positive, that it will solve our issues. But that's long-term, and in the meantime we might screw things up for a lot of people.