I object to the notion that the greatest threat humanity faces is lack of fecundity.
Humans have problems - but fecundity is not one of them.
Looking at the graph of human population over the last few thousand years it seems obvious that we are headed up for 8 billion a lot faster than we are headed down to 6 billion.
I also don’t think overpopulation is that big of a problem - IF you have a corresponding change in technology that makes us all live in health and comfort. I think the Earth could easily support 14 billion people - but not with present day technology.
People see a trend and then say “What if...... what if the trend continued unabated for the next forty years?”
The recent decrease in birth rates from historic highs will most likely be a dip at best that will slow but never decrease our rapid climb to 8 billion people.
The sky is not falling. In forty years the sky will almost certainly not fall. When children are needed women can produce them at a rate of 7 per woman.
One thing I think we can all count on is human beings fornicating and producing children when they want them.
In forty years if more children are needed I am sure there will be volunteers to make them.
Until then we are at 7 billion and rapidly approaching 8.