Earth will have another 10,20,30 Billion people living on it
Luckily, we're looking at a very real possibility that we peak at 9 billion and then start to shrink from there. Which creates a whole host of other social issues, but at least gross overpopulation isn't one of them.
The shifts in the average age and ratio of workers to retirees in Japan and America are absolutely sudden on societal level. 10-20 years is very long for an individual, but it's lightning quick otherwise.
That's at least a 30-40 year trend that everyone who studies demographics was aware of, not a 10 or 20 year trend that suddenly came out of nowhere. Japan's population stopped increasing meaningfully 30 years ago. There's no surprise or sudden jolt to their population declining over the coming decades. They'll have had a half century warning on the problem by the time meaningful decline sets in.
As a recent example, by the time China's population begins flat-lining on growth and or declining in an estimated six years or so at the current trend, we'll have known for 35-40 years that China was set to age rapidly.
30-40 years is sudden for societies. Just because individuals can predict in advance doesn’t mean that the society at large will successfully absorb the enormity of a change and then adapt to it. Hence, sudden.
For what it’s worth I think we’re still dealing with the destabilizing shock of the baby boomers outnumbering other generations, and have been dealing with that since the late 1960s.
Luckily, we're looking at a very real possibility that we peak at 9 billion and then start to shrink from there. Which creates a whole host of other social issues, but at least gross overpopulation isn't one of them.