Total population dropping at the present time is not the point. The concern is population vs age distribution.
A decrease in birth rate can result in a massive shift in the ratio of people working vs retired. Depending when the rate was higher, there can be a looming issue that isn't reflected in total population.
In 2020 there were currently about
2.6 billion in the -19 bracket (1.13x older gen)
2.3 billion in the 20-39 bracket (1.3x older gen)
1.8 billion in the 40-59 bracket (2x older gen)
0.9 billion in the 60-79 bracket
0.15 billion in the 80+ bracket
Using rough ranges for simplicity.
Roughly 4.1 billion in the working range (20-59) vs 1 billion in the retired range, a ratio of 4:1.
Shift out 20 years and it's 4.9 billion working vs 2 billion retired 2.5:1 or a 60% increase
If annual births stay flat, in 40 years it would be 5 billion working to 2.8 billion retired, 1.8:1, a >2x shift from present conditions.
If poulation only stays flat (massive drop in births), in 40 years it could be 3.5 billion working to 2.8 billion retired, 1.25:1, over a 3x shift.
Numbers from
Visualizing the World’s Population by Age Group