Why is it a crisis for populations to decline to levels of the first half of the 20th century? The world worked just fine back then with that number of people.
There are problems that arise from a population that contains a lot of old people, but that's a problem that fixes itself in a few decades, and balance will be restored.
Pick one crisis: no jobs, or no people.
stupid question: if by some magic 80% of the population of the planet disappeared tomorrow, what ll happen to the economy and stock market
The age distribution is most of the problem. It doesn't help to have a lot of people who need somebody else to be working at the hospital for them. If you skip straight past looking at the period if imbalance the situation looks a lot better.
The same number of people, but on the way up and growing, compared to on the way down and shrinking, is very different.