I think you missed the number 1 cause for population decline; women can now control their own fertility. This technology did not exist in the past. When a new technology enters society there is always disruption, and we can see that clearly with birth control and easy access to abortions.

What society needs to do is cater to, and encourage women to have children. Otherwise population collapse will be a very real possibility. I think we may see the return to a single income family, and maybe men staying home more and raising children. I think we will see a huge cultural shift in the next 20 years as the prospect of population collapse will ripple through the world economy.

As Elon Musk said recently, population collapse is a very real danger and actually more dangerous for the human race than climate change.