In North America / Europe, you’re correct. In Japan, China, South Korea, they’re losing population, everyone knows they’re losing population, but self-correction mechanisms aren’t working or not working fast enough. Japan been trying to throw random stuff on the wall to see if anyone sticks, but the birth rate only keeps dropping. And well, young people really don’t want children.
It all depends on what do you mean by "fast enough" - the self-correction mechanisms would trigger after the population has shrunk, not when it starts shrinking; and it would apply to the generation after that. Also, we might consider the pressure from housing and jobs as a correction mechanism for overpopulation, so it would likely stabilize somewhere much lower than the peak.
While "everybody knows that they're losing population", South Korea and China haven't even started shrinking (they've stopped growing, and have been at approximately 0 population change for the last few years, it's technically negative but the change is insignificantly tiny) so obviously any self-correction mechanisms wouldn't apply at all yet.
Japan's population reached its peak in 2010 and as of 2016 the shrinking was quite low - so we're talking about just like 7 years of actual shrinking and that's not that much time to cause any socioeconomic changes triggered by population reduction, it would seem reasonable to look at the generation which grows up in a shrinking-population regime to see how it affects them, when they actually have real estate that gets significantly cheaper due to lack of population and scarcity of workers pushing up the bargaining position of labor; Japan's demographics is "leading the world" in this regard, but even they aren't there yet. And they have time to see how it works out - Japan's population is still far above its e.g. 1950 population, and continuing the current population trends it would be as long as 2075 to reach the 1950 level (which isn't bad!) again.
I agree with you about how we probably haven’t reached the point where the governments are supposed to worry, but I don’t think looking at a point in time in the past is right. Even though we have a long way to go for the decline to reach the population levels of 1950s, the demographic pyramid is pretty much inverted. Same applies to China and South Korea in some degree. Most of the women after the age of 40 aren’t going to have a kid, yet alone 2.1+ to revert the population decline.
But you’re right, more drastic measures will be taken in political and economical senses once absolute numbers start ramping up. It’s fairly obvious our current system isn’t the best for aging population with not as many children. Unfortunately, we just haven’t come up with anything better that gives some sort of stability and peace.
