Imagine if the Earth's population fell to just two billion people over the course of a century.

A century ago, the earth's population was two billion people.

Then there would be 1.5 billion people above the age of 65 and 0.5 billion below it. That seems problematic.

If you really wanted to get to 2 billion people, better to do it over a thousand years than a hundred. But is that actually good for anything? Having a hundred billion people would probably be problematic, but we seem to be doing alright with 7.5 (once we stop burning oil).

If all 7.5 billion people want to live in the prosperity of upper-middle class US or North-Western Europe, that may be a problem: either because they get it (and the planet becomes inhospitable for mankind) or because there will be conflicts (see current migration issues) due to the inequality that is now broadcast by a world wide communication network.

So one way to keep or raise the standard of living for everyone is to have fewer people consuming resources for it. The most humane way is voluntarily not creating new kids and hoping that this helps quickly enough.

> If all 7.5 billion people want to live in the prosperity of upper-middle class US or North-Western Europe, that may be a problem: either because they get it (and the planet becomes inhospitable for mankind) or because there will be conflicts (see current migration issues) due to the inequality that is now broadcast by a world wide communication network.

If you expect them all to be buying coffee pods every day and non-repairable epoxied electronics, sure. But is it really that much worse of a lifestyle if your coffee comes from a five pound bag instead of a half ounce pod and your computer comes with a battery that can be removed, recycled and replaced rather than having to buy a whole new computer every time?

> So one way to keep or raise the standard of living for everyone is to have fewer people consuming resources for it. The most humane way is voluntarily not creating new kids and hoping that this helps quickly enough.

It's a bad way for a number of reasons. It creates all the usual problems with population decline and a declining ratio of productive workers to retirees. It takes an entire generation to do anything at all which means it can't help any of the people who are currently alive (whereas things like repairability and renewable energy that can help us can equally help our children). It increases wealth inequality because without intervention the fertility rates are lower in wealthier regions, compounding their advantage by distributing their parents' affluence over a smaller number of children. Childless affluent people also have terrible incentives to mortgage the future of others' children for present-day benefits, and they have disproportionate resources and influence because they're not spending their wealth and time raising the next generation, so increasing the number of such people is quite problematic.

> because they're not spending their wealth and time raising the next generation

Adoption (providing care for children that are already there), education (providing a better future for children that are not your own), foreign aid (providing a future for children that live in poor regions) are all ways to raise the next generation without contributing to the population count.

They are also all things that require altruism directed toward complete strangers, which on average people are much less inclined to do in practice than doing the same for their own children. Even high-minded people who think well of themselves and donate money to such charities -- hardly anybody gives as much to the charity as parents give to their own children.

A counter-question: imagine the Earth's population is just 2 million. How many high-technology jobs would we have with the current level of robotics? How many Einsteins would we have?

The [optimal] population size is directly linked to the current level of technology. If it feels like humans would be better with fewer population it only means that our technology and resource utilization skills lag behind and the solution should be to advance in technology/management, not reduce population.

They’d have to go low across the whole planet, and stay low for a long time to be worrying given the billions of people alive today.

TBH, I don't understand the argument about whole planet. Some nations (or species) can go extinct on their own, while the whole planet doing just fine.