So, how did the humans manage when there were only 1 billion of them?
It is not the number, Rich.
Before pensions couples tended to have large families because your kids WERE your pension.
That is still true in many countries that don't have pensions.
Since that need has been taken away people are having less children,less than 2 per couple in some countries.
We need kids to support and pay for the pensions of people who are living longer and longer.