Demography is in some sense the
primary driving factor behind an economy. The ratio of working people to the unproductive cannot stray too low, otherwise you run into the double whammy of economic problems where fewer workers sustain the pensions of more elderly and political problems where the elderly, who wield more political influence, vote to increase their own pensions (or just never decrease, which effectively raises taxes on the working as fewer of them support more elderly) at the expense of the working. The key indicator here is the "age dependency ratio".
> First off, if we want to bring the birth rate back up, we would probably do well to implement social policies that makes having kids less of a nightmare for everyone but the very rich or very poor: generous paternal leave, some kind of childcare (a creche-like system), better tax credits, good public schools, etc. I understand that well-paid developers might be less attuned to this kind of thing, but as a non-dev professional in a HCOL city with a young baby, I promise you that economic factors are the major reason we will not have another child.
Oh boy, tell that to all the developed asian economies. What you're describing is known as pro-natalist policies, and many east asian countries have tried (and failed) to raise fertility by implementing them. South Korea, Japan, Singapore, Hong Kong, all have varying degrees of pro-natalist policies, none of which have been met with much success. I think there's a case to be made that their demographic problems would be worse if they didn't implement these policies though.
> Second, while I understand that the economy is organized around ever-growing populations, it seems like at some point, we'll have to figure out how to deal with declining or flat birthrates. We can't just grow forever, can we? Could the planet really sustain triple or quadruple the current population?
Quite trivially. America alone throws a huge portion of its perfectly good food into making biofuels and feeding livestock; the primary constraint on food is not that we can't grow more, it's that it gets more expensive on the margin. When farmland runs out, you move to hydroponics, then to aeroponics, and those can grow 10x the food in the same amount of space. Water from the oceans is effectively limitless, desalination is just more expensive. Not everybody can have cars, but that's why you build public transit. And on and on it goes. Resources are not finite, it's not a video game where earth has 1000 units of iron and that's it. They just get more expensive on the margin, which then shifts consumption patterns to establish a new equilibrium. Recycling starts becoming profitable, some metals get too expensive and are only used in high-end applications, etc.