Is population collapse due to low birth rates a problem?

The only hard limit on our resources that we currently know of are the energy output of our sun in the short term and the energy output of reachable stars in the long. We are not constrained on food, space, energy, etc on earth in the short term. We have only artificial political constraints where the stationary bandits ruling the various geopolitical units have subsidized the creation of narratives justifying your constraint. Humans do not consume wealth, they create it.

I think the tweet is saying a lot less than it seems like it is. 

In particular, the risk of civilization collapse due to climate change is really small. There are all kinds of bad things that might happen due to climate change, and I'm strongly in favor of working hard and spending a lot of resources to fight it, but I think it's < 5% for even business as usual to lead to civilizational collapse due to climate change.

Suppose Musk thinks that as well -- and in fact probably his estimate is lower than mine for collapse. Now he's only needs to think that there's a small chance of collapse due to low birth rates for it to be greater than the civilizational risk of climate change.

It’s true that more people means we each get a smaller share of the natural resources, but more people increases the benefits of innovation and specialization. In particular, the benefits of new technology scale linearly with the population (everyone can use the) but the costs of research do not. Since the world is getter richer over time (even as the population increases), the average human is clearly net positive.

This model is wrong because it doesn't take into account the enormous gains to productivity you get from various kinds of economies of scale, particularly from the possibility of specialization and trade between different people.

Even if you had all of the technical knowledge we have right now, a town with 1000 people would be much poorer on a per person basis than we are now. There would simply not be enough people working on enough kinds of different jobs to make most of what makes the modern world possible work. For example, how do you build a modern laptop with only 1000 people? Even if all 1000 were exceptionally talented, that's just not enough scale for you to be able to produce a laptop.

In addition, there are large positive externalities from having additional people, because a lot of research is high fixed cost and small marginal cost. It doesn't cost ten times more in research effort to make ten rockets compared to making one rocket. So the more people you have who are working on figuring out how stuff works and spreading that information around, the more productive everyone becomes.

You can think of this as a tradeoff: the more people you have the more you're able to produce per person due to the effects mentioned above, but the less natural resources you have per person to go around. If you view this as a purely static problem then there would presumably be some "optimal level of population" at which income or wealth per person is maximized, so the question is how big this optimal level actually is. I think all available signs right now point to us being far below the level of population that would be optimal: the bottleneck on our productivity and economic growth is not resource constraints but the lack of enough people.

To sustain high tech-driven growth rates, we probably need (pre-real-AI) an increasing population of increasingly specialized and increasingly long-lived researchers+engineers at every intelligence threshold - as we advance, it takes longer to climb up on giants' shoulders. It's unclear what the needs are for below-threshold population (not zero, yet). Probably Elon is intentionally not being explicit about the eugenic-adjacent angle of the situation.

In my model of the world works, overpopulation is in fact a big problem. In general, the more people you have, the less resources you have to share among those people. A decreasing population would be in fact good news, although maybe not in the short term.

That is true in general, but we are nowhere near the resource limit currently, provided we manage them properly. There are existing low- to high-tech measures, like reducing cattle population, improving efficiency of existing HVAC systems, switching to electric, GMO crops, improved renewable and nuclear energy sources, smaller dwelling footprint per person, and many others. This doesn't include any new tech, like fusion, vat meat, energy beaming from space, moon bases etc. 

With an eye on conservation of resources, without giving up quality of life, the planet can probably easily support an order of magnitude more people than there are now.

Most of the world is covered with people. Economy and its productivity cannot be measured today. The measures or measuring stick we use to explain the world today is inadeqate. Retrospective analysis of humanity is nice. But very risky business since nothing today ressembless the past.

I have a terse answer: If you're not growing, you're dying.

Somebody has to support the elderly people. The other aspect is the capitalism and consumerism point of view. But all this hopefully will be good for the planet and other species.

we need ideas to have econ growth, we need humans to have ideas, having fewer humans will lead to econ stagnation. This is the summary of the argument https://www.cold-takes.com/what-is-economic-growth/

Note that decline is not collapse is not extinction.

https://astralcodexten.substack.com/p/slightly-against-underpopulation

If human lives are good, depopulation should not be pursued. If instead you only value avg QOL, there are many human lives you'd want to prevent. But anyone claiming moral authority to do so should be intensely scrutinized.

Isn't he just trying to win points from his new republican buddies by disowning earlier interest in climate change and aligning himself more with a cluster of socially conservative views (anti abortion, love of big patriarchal families, fear of being outgrown by the out group etc)?