I am intrigued and alarmed by recent reports on your comments regarding ‘population collapse’—the notion being that we risk economic catastrophe if we do not take measures to increase the number of earthlings on this tiny blue planet.
A quick Google of your Twitter comments show you saying things like: 
- “A collapsing birth rate is the biggest danger civilization faces by far”
- “Population collapse is 2nd biggest danger to civilization after AI”
- “We should be much more worried about population collapse”
- “Population collapse due to low birth rates is a much bigger risk to civilization than global warming”
In fact, PopulationMatters.org has compiled an even longer list of your proclamations: 
- “The world’s population is accelerating towards collapse.”
- “The biggest issue the world will face in 20 years is population collapse.”
- “Earth could maintain a population many times the current level.”
- “UN projections are utter nonsense.”
- “Let’s not gradually dwindle away until civilisation ends with all of us in adult diapers, in a whimper.”
- “If there aren’t enough people for Earth, then there definitely won’t be enough for Mars [sad face emoji]”
- “We’re going to be fine even if we doubled the [population] size of the humans. I know a lot about environment stuff.”
These are bold statements. And I sincerely hope that you are not drumming up fears simply to sell more electric cars in the future. I hope you will someday elaborate on your views about population collapse. Perhaps you will accept the invitation to debate the matter with the Population Matters Director Robin Maynard? 
Personally, I have not seen enough evidence to convince myself that population collapse is imminent. For starters, the UN Department of Economic and Social Affairs’s median projections expect the world population to grow at least until 2080.  But even if that was not the case, there seems to me to be a lot of untapped potential and latent capacity in the global workforce. Underemployment, for example, remains a challenge in many parts of the world. [4.1] This is especially so among young people, women, minorities, and marginalized communities. Moreover, an unacceptable fraction of the world continues to live in poverty. While making more babies might improve things, wouldn’t it be more prudent and morally just for society to lift the participation and productivity of the most disadvantaged first?
In my opinion, it is lazy to assume that we can simply outgrow our problems. Frankly, there is a lot of waste and useless work inside our government and corporations right now. Few people would disagree with that. Sometimes, we just need to find ways to do things smarter, leaner, and fairer. Elon, you should know this better than anyone given the portfolio of incredible companies you’ve built. We don’t have to overcomplicate things with grand visions of demographic transition and population controls.
Yes, an aging population will imply more public and private spending. Social security, age pensions, and other big expenditures will certainly rise. But we should not forget that spending must go somewhere. In this case, it will be to the pockets of producers and laborers in aged care, healthcare, hospitality, entertainment, tourism, and other industries that serve our future retirees. You may or may not be right about the downsides of population aging. But income flows and wage adjustments are complicated procedures. They deserve careful analysis before sweeping statements.
What’s more, we have to look for confounding factors and interactions. What happens when artificial intelligence, advanced manufacturing, synthetic biology, quantum computing, and other paradigm shifting technologies grow manifold? How does that interact with a shrinking or growing population? I’m not too sure. But many people today are fearful that there will not be enough jobs for people because of automation. Yet many people are also fearful that there won’t be enough people for jobs because of population collapse. Can both be true?
I should remind you that not long ago, people during the 18th, 19th and early 20th century called for smaller populations and lower growth rates. The argument was championed by the economist Thomas Malthus who warned that exponential population growth would eventually outstrip our finite food supply.  What he did not foresee, however, was the boons that technologies (e.g. fertilizer) and industrialization would bring to food production, and the change in fertility rates due to contraception, demography and other factors. Perhaps we are all on a perpetual seesaw. If our population multiplies manifold as you desire, others will scream “Malthusian crisis”.
One issue, however, is that of relative economic, political, and military might. By sheer scale, larger countries with more people may produce more inventors, industries, and output. And a larger economy implies a more powerful military and voice in geopolitics. I too find this argument compelling when I wear a nationalistic hat. But one must wonder to what end this global arms race will take us. And we should remember that the total number of heads and mouths is not the only solution. There are always alternatives. Strong international alliances, collaborations, and exchange, for example, seem to me to be more essential for preserving the balance of relative powers.
I should admit, of course, that I do not know what will happen. All I really want to say is that the dynamics and interplay between population and wellbeing is complex. We need to trace the feedback loops, externalities, general equilibrium effects, unintended consequences, and policy responses. And these problems extend well beyond first- and second-level thinking. Here, ‘A implies B’ is an insufficient model of reasoning. What we have here is a web of interacting variables. Most of us, myself included, are not in a position to reason about such systems in our heads. Economic and demographic modeling, while imprecise and sometimes incorrect, is indispensable here.
Elon, I know you are unlikely to have the time to enlighten me. But I hope you will take a moment someday to clarify your position and thoughts. In return, I will dedicate some time to educate myself on the population issue too.
Take care and warm regards,
 Elon Musk’s population collapse claims reckless and flawed. <https://populationmatters.org/resources/which-planet-he-elon-musk-and-population-apocalypse
 Elon Musk’s population collapse claims reckless and flawed. <https://populationmatters.org/report-elon-musk-population-collapse-claims-reckless-and-flawed
 UN World Population Prospects 2022. <https://population.un.org/wpp/Graphs/Probabilistic/POP/TOT/900
[4.1] Underemployment as a global challenge in the 21st century <https://www.researchgate.net/publication/325823327_Underemployment_as_a_global_challenge_in_the_21st_century
 Malthus. An Essay on the Principle of Population. <http://www.esp.org/books/malthus/population/malthus.pdf