Arguments by smart people like Sabine Hossenfelder have led me to question the conventional wisdom on population growth.
For example, see https://www.youtube.com/watch?v=VI1AaZ9OkH8 .
dPop / dt = fertility rate * pop (birth rate) - deaths
Note the "current population" factor in that. As long as the number of births depends on the current population (i.e. anyone has a chance to give birth), this equation will result in an exponential curve. If the fertility rate is greater than replacement rate (that "deaths" term that I've handwaved away), the population will exponentially grow. If it's less than the replacement rate, the population will exponentially decay. But it is necessarily exponential, because it's a constant rate multiplied by the current population, and if you remember your calculus, an exponential is defined by a constant growth rate.
There's a whole subfield of population dynamics, and I remember lots and lots of predator/prey models, resource constraint models, etc. from my applied calculus class. Some of those do have non-exponential growth curves, eg. a basic 2-species predator/prey model gives rise to sinusoidal population curves as the increase in prey leads to an increase in predators, which results in more of the prey being killed and eaten, which results in a decrease of food, which results in the predators dying off. But what they all have in common is death rates that are proportional to some function of the current population. In other words, you have to kill off proportionally more of the current population based on how many people they are, something which would be ethically unacceptable to most humans. If you just have natural deaths (i.e. a death rate proportional to Pop(t - life expectancy)), you always get either exponential growth or exponential decay, because that's the way the math works.
I suppose there was one other population curve that gives a sigmoid function (i.e. a logistic curve that asymptotically approaches a limit but never reaches it). This is what you get when there is a natural resource limit to giving birth, i.e. when instead of births being fertility rate * total population, it's fertility rate * (exponentially decreasing fraction of the population). Many people find this scenario ethically challenging as well: in plain English, it means that only the elite can afford to have kids, or alternatively - cap the number of houses so that only the top X families can have a house and raise a child, and then create social stigma around having kids when you cannot afford those increasingly scarce markers of stability. It is disturbingly close to reality, though.
Economists like to pretend that everything is a continuous function of price, but this very rarely reflects real markets. Most utility functions are step functions; there is some point at which changing your behavior becomes economical, and above it everybody switches, while below it nobody switches. People can either afford to raise a child in the lifestyle they expect, or they can't - and many of the sacrifices in lifestyle don't move the needle in fundamental living costs. For having a child, that point appears to be in the hundreds of thousands of dollars. If you make childcare and college free and give 2 years of maternity leave and ensure that you can buy a 3BR+ house on an average income, then sure middle-class people will start having kids again. All the middle class people will start having kids again, because they all face the same economic constraints. And then you'll have fertility > 2 again, and the population will start exponentially growing.
You could have a large per-child subsidy and cap at 2, but this also is a lot harder to generate the desired effect than expected. Accidental pregnancies are a thing. Twins are a thing. Infertility is a thing. Careers are a thing. People who want to remain childfree regardless of cost are a thing. If you cap at 2, you'll get a fertility rate under 2 because of all these other considerations.
And this is what's playing out empirically across many countries right now. A lot of European countries have generous subsidies for childbearing; they still find that they can't raise the fertility rate above 1.5 or so. China lifted its one-child only policy, moving it to 2 children in 2016 and 3 children in 2021, and finds that fertility still collapsed (even after instituting subsidies) and they can't convince people to have babies. There are significant non-monetary costs to parenthood; it's a whole other lifestyle, and the money involved needs to be pretty large to alter that.
I am not accounting for immigration of course, but from the wider point of view, immigration is musical chairs. Sri Lanka is well below replacement and still falling fast. Who is going to immigrate there? Ditto for most other countries.
I think it mostly has to do with increasing desires for a high standard of living when it becomes possible and the pressure of high density cities making lots of kids an unattractive prospect.
A rapidly shrinking population is not ideal.
Steady state population should be the long term goal, at least until we start moving into orbital colonies or something. But unfortunately that's an anathema to growth oriented economies which makes it the bad guy in economics and politics.
The population only went over 2 billion in 1927. Dropping from current numbers is fine, and less competition over space would be nice.
Not to mention the changing political dynamics of having a lot of old people outnumber younger people by 2 or 3 times in the voting booth. The gerontocracy is bad enough now. Imagine it 3x worse.
https://www.indexmundi.com/graphs/population-pyramids/united...
Looks fine to me. What's your definition of "old" here? Are you specifically worried about Korea like you mentioned in another post?
I'd worry about the implications of a global 1.6 birthrate if they were actually going to happen. I don't think that's very likely, though. I don't know if we're even going to get down to 2.
Unless someone thinks of a democratic, scalable solution it's going to be a lot of pain for a lot of people for a long time.