"artificial intelligence raises the spectre of machines replacing humans at virtually all jobs so that parents do not want to have children who will have dim prospects in life."

AI is fairly obviously not yet a cause. Maybe it will be, but it will also be part of the solution, in lowering the cost of raising a child.

What you completely ignore is how government has replaced family and church as the social safety net. Today there is much less reason to get married and have children. That is reversible, at the cost of some disruption.