Nothing will ever beat the whiplash of society saying
“STOP HAVING KIDS!!! THERE ARE TOO MANY PEOPLE!!!” When I was 13
To now saying
“EVERYONE HAVE KIDS!!! THE BIRTH RATE IS DECLINING!!!” at 20
My people, make up your minds
People can change their minds when they have more information.
Overpopulation is an issue because of resource scarcity. If you can't take care of everyone, then things are going to get messy. People might work together on the small scale (i.e. a neighbourhood working together to help each other), but on the larger scale (i.e. a whole country), then people will fight for and hoard resources. If violence didn't quickly restore the population to a more managable level, then famine and disease would be rampant as people stop sharing--if they give someone else their food or medicine, it's not like they can go out and get more, because there just isn't enough to go around.
An aging population is an issue because they're not even producing at replacement levels is also an issue, especially for social species like humans. Assume 80% of the population just stopped having children. Then, when they get old and need assistance, who is taking care of them? And I'm not talking about individuals feeling some sort of obligation to take care of their aging parents or in-laws--even nursing homes would be strained with more and more people needing to live in one but no one working there to take care of them.
Fun fact though, the reality is that most countries aren't facing either issue. Women aren't necessarily having fewer children, they're just having them later in life. Old measures were counting how many children women had by the time they were 30--when her first was born when she was in her early to mid 20s, then it was reasonable. But it falls apart when a lot of women are having their first kids in their early 30s instead. Now, if you look at how many kids a woman has had by the time she's 40, then the number will be about the same as what it was 50 years ago. And yeah, that's a pattern that's going to take longer to be noticed because it has to actually happen first. That's the challenge with measuring long-term data in real time instead of looking at it decades after the fact.
Also, in your comment about China's One Child Policy, was not intended to decrease the population. It was intended to slow growth. Growing by ten million people one year and one million people the next is not a population decrease, it's just slower growth. It was enacted because of the aforementioned resource scarcity concerns. At the rate the population was growing, they would be unable to feed themselves very quickly; slowing the population growth would give production and infrustructure a chance to catch up.
As an unintended consequence, because of the expectation that a couple would care for the husband's parents in their old age but not the wife's, couples would rather have a son because a daughter would mean no one would take care of them when they needed it. It led to a lot of abortions of daughters, and baby girls being abandoned (for decades in Canada, it was literally cheaper to adopt a little girl from China than to adopt a Canadian child because of the insane number of Chinese girls that weren't being kept) so the parents could try again for a son. Within a couple generations, the gender ratio was fucked. The policy was lifted to try and balance things out again, hoping that couples would aim to have a son and a daughter.













