To be clear, I don't think this is the business of governments and nobody should be coerced to have or not to have children.

I believe that in case of population collapse, the solution will naturally appear to large enough number of people, evolution shaped us to reproduce, I don't think this instinct is likely to disappear, even if we might not fully understand it and why girls and women tend to have less children in some conditions (it happened before, in antiquity, and was considered to be a problem by Roman politicians, many of the current traditions around marriages were first created as laws...)