I do tend to throw a lot out, and try to limit myself. I'll stick to the first point here, with some further expansion on my thinking.

The first time I'd run into the notion that high metabolic systems burn out faster was reading Asimov's book on black holes, The Collapsing Universe, when I was a young boy, and learning that the hotter and brighter a star, the shorter its life. The very largest known star, CY Canis Majoris, is a red supergiant, near the end of its life, and is thought to have formed 8.2 Mya, roughly the same time h. sapiens and chimpanzees diverged on the evolutionary tree. Contrast the birth of our own Sun at about 4.5 Bya.

The notion that human civilisations might be somewhat akin to stars, burning through their metabolic and resource foundations, has a symmetry to this concept, though there are more confounding factors to cultures than there are to starts, where metallicity, mass, and the presence or absence of stellar companions are about the only relevant factors affecting lifespan.

The idea that civilisations are bounded by limits and that growth only reaches those limits more quickly also punches some gaping holes in the notion of long-termism, which argues for maximal growth and growth rates. That makes sense if ultimate growth and all stages along the pathway are both unlimited. If, however, there are both local and global maxima, that is, ceilings to growth and sustainability, and most especially if reaching those limits is a high-risk event, then rather than argue for maximal growth at all times, a more reserved approach is called for.

The question of two species or cultures competing for the same resources is another point, and it's been raised several times in this thread. I'd look to actual ecosystems where fast- and slow-growing species (including r/K selection theory: <https://en.wikipedia.org/wiki/R/K_selection_theory>) exist. If the slower-growing species is also the more resilient for other reasons, then its slow growth need not be an ultimate handicap. That of course isn't a given, but depends very much on both the fitness landscape (selective pressure) and specific species adaptations. Human cultures are not identical to biological species, though they share some resemblances. Most especially, cultures are malleable, individuals can move between cultures, and the practices and knowledge of cultures can be shared, adopted, and transmitted to a much greater extent than biological species' genetic code is.

(I'd also like to make as painfully clear as I can that I am NOT equating "human culture" with any specific ethnic or geneological line, but rather the knowledge, practices, language, geographical distribution, and activities of groups of people, whether of common or diverse ancestry. Rather I'm focusing on both as persistent patterns which share the characteristics of inheritance, variability, and selection, and are thus evolutionary phenomena.)

As discussed in another comment, alliances matter far more for cultural protection than measures of wealth (collective or per-capita GDP, say): <https://news.ycombinator.com/item?id=37400918>.

I'm not saying that national capability as expressed in defensive capabilities doesn't matter at all, but suggest that it matters far less than one might generally think. Even at times where marauding empires were fashionable, the empires they created rarely survived long: Alexander, Rome, Napoleon, and Hitler all made tremendous conquests, but in three of those four instances, lost control within a few years, decades at the outside. Rome itself held at maximum extent with no peer in capabilities for a few centuries.

Common interest and fraternal bonds seem far more effective than military might. Certainly less costly. And one could argue that both Pax Romana and Pax Americana were based more on economic than military factors.

On the 2nd and 3rd points: Certain forms of growth, most especially those based on fossil fuels, must end in the very near future to avoid absolute catastrophe. The political and economic opposition to this remains strong, unfortunately. As for channeling growth into basic infrastructure and safety nets, amen!

Glad you're getting something from this.