I enjoyed reading your commentary but didn't find that it supports your musing here: "I think about the Fermi Paradox. What if the answer to why there are no galactic civilizations is as simple as "because capitalism"? "

That wouldn't make any sense, because if it were true then we wouldn't have seen the population explosion take off under a primarily dominant global capitalist economy in the first place. The ability to distribute goods and services in a decentralized way and accompanying technological advances which are supported by market opportunities focused on profit have paved the way for the 8+ billion people on the planet who are almost all living in much better conditions than most human beings throughout history. I'd argue if anything capitalism enabled overpopulation, not the inverse.

With respect to birth rates I believe there are a lot of factors at play, but one which is perhaps the largest is the substitution of the family unit with the state. Even as little as 100 years ago we were a lot more family-oriented with much of that seeming to dissolve recently. We don't need a child or 5 to take care of us when we're older, we have a 401k and an HSA for that. There are pros and cons there.

The mix of factors influencing declining birthrates I would argue depend on the country. South Korea is a homogenous country of 50+ million about the size of Indiana. There are too many people for too few opportunities and everyone wants the good life. That's why people are spending their life savings on tutors for their children, unlike the United States perhaps where while we have quite a number of people living we are the world's most diverse country and here we have (for now) enough opportunity for folks to pursue careers, start new businesses, and such.