Most societies have reached a point where fertility rates have declined to below replacement.
It seems reasonable to assume kids having more positive economic power in the past played a significant role, but there's counter-examples abounds in modern times. Nigeria is heavily urbanized yet has one of the highest fertility rates in the world. Or on the other end of the spectrum, Thailand has a very large chunk of people living off farming, yet has one of the lowest fertility rates in the world.
[1] - https://en.wikipedia.org/wiki/List_of_countries_by_total_fer...
And all the while this is happening your economy is getting wrecked not only because suddenly you have far fewer people, but because your society turns into a giant retirement home, as fertility also determines the age ratios within a society. You can see this by working backwards. Again taking a fertility rate of 1 as an example, if we have 1 newborn then we have 2 20-40 year olds, 4 40-60 year olds, 8 60-80 year olds, and so on. So you have a shrinking economy paired alongside an even smaller ratio of workers, and much greater social strains on that economy as well.
This is also why immigration isn't anything more than a very short-term bandaid. When you're speaking of population declines near 100% over such small windows of time, you end up needing to basically replace the entire population of a region. Doing this with immigration, let alone as this is increasingly a global crisis, just isn't realistic. And this is completely ignoring the social consequences of such actions.
It's only really sub Saharan Africa still growing.
And governments in those countries are importing as many people as they can to paper over their own population decline.
In the past most societies had high fertility rates, which means there are relatively few elderly, relative to younger people, today. So the global death rate is relatively low as the the number of people annually exiting the stage is relatively low. But when low fertility generations start to die off, this gets turned upside down. You'll have large numbers of elderly dying off, and they'll make up the majority of the population. But this only happens ~80 years after the birth of the low-fertility generation.
This is why Japan looks so catastrophic today even though their fertility rate is much higher than places like South Korea. They've had a mediocre fertility rate since the 60s which then went into collapse in the 70s. By contrast South Korea had a fertility rate of 6 in the 60s, and didn't fall below replacement until the 80s. So Japan has had a solid decade or two head start. The fact we still aren't at the life expectancy of the first below replacement generation in Japan also means today is still going to be seen as 'the good ole days' for them. And this is where most countries are headed.
If the current generation has a 1.0 fertility rate, that means the next generation will be half as large. If said generation now doubles it's fertility rate to 2.0 it will only maintain the size of that generation, all the old people will age out and be gone all at once.
Now you might say, fine, let's fix it, 4.0 fertility rate, that will only double the size of the next generation, meaning it will only fix one generation of 1.0 fertility. You have stabilized the population, not increased it.
Even if you do this, you now run into a huge bottleneck: parents need to take care of their old parents and their large family all at once. You fixed the biological problem but you're now running into economic collapse.
You convinced me!