Decreasing population is a function of increasing prosperity.
Prosperity includes increase in literacy and general education levels of the population. Most notably, women will have less children and shift towards actively taking jobs, opening businesses, research, civics,...
Lower birth rates and an ageing population then become a challenge to tackle.
Those who age stop actively working - adding direct economic value as taxes or as marketable goods & services - and their income is replaced by a pension provided by a public social security system and/or a pension fund they contributed to.
Now, this is where some will throw their hands in the air and yell "Doooooommmm! A dwindling active workforce can't possibly create enough wealth to sustain the pensions of those that don't!". hey then go on predicting how entire economies will collapse and how everyone will end up becoming working poor.
But that narrative hides tons of naive and downright wrong assumptions.
Like, how literally everyone should, can and will retire at 65 and spending their golden years on long holidays. Or, how it's impossible for anyone over 60 to be as productive or flexible as a 20 year old. Well, you can come up with a ton of other stereotypes.
In reality, tons of people over 60 or even 70 are able and willing to work and contribute to society. For instance, guess who makes up for a lot of the volunteering force in charities, public services and (libraries, archives, social work,...)? Young retirees. These are important fields that are publicly underfunded, and so retirees take up an important place here.
But you also find seniors taking up jobs again simply because their state pension is insufficient to make ends meet. That happens too.
The big challenge in this isn't the elderly population, it's adjusting our work culture to include older age cohorts. Because right now, workfloors and work in developed nations is entirely geared towards young people in their 20s, 30s and 40s.
The other big challenge is wealth inequality in general. As long as the system is geared towards extracting wealth to the top at the expense of the workforce - labor rights, minimum wage, healthcare,... - it's only to be expected that older people will drop out earlier rather then later.
So, that's basically what we're looking at. If you can balance an economy between sustainability and prosperity, you also solve the problem of population.
Now, guess what you need to do that? A proper, healthy social democracy that puts humans as their top priority. Not the interests of a few elites.
In a perfect scenario you’d just have people have fewer babies. The issue is that countries want growth at all costs - the incentive is always more, bigger, faster.
Current forecasts are than we'll add about 3 billion human beings before population stabilises.
If you also take into account that the majority of human beings are poor by Western standards you can see that, IMO, sustainable global prosperity is likely impossible with these numbers, not least if we want to keep at least some wilderness on this planet.
Europe, Japan and the native population of the US have elected themselves. Africa probably won’t have the population boom since education is spreading, which tends to slow reproduction rates. Problem is solved in about 30 years with the death of the boomers and the non-replacement rates.
What worries me is will we do enough damage in 30 years that it doesn’t matter? And will economies be able to adjust? Even if this happens, do we get to a steady state of 10bn? 9? I think our global population should be closer to 1bn if we want an ideal world.
Also boo to those who are downvoting you about the US comment. Either you can’t read or you’re intentionally not reading. The OP was referencing people who are citizens in the US who do it have as many kids as immigrants and without immigration would be below replacement rates.