You don't. People should have more kids, period. What people don't get about the declining birthrate is that it leads to a graying of areas and a huge economic malaise as the elderly consume more and produce much less in terms of resources. This is part of the reason places like Japan or Italy are struggling so hard...they don't have enough native workers to support their aging population.

The demographic death spiral can be real. And if anything, there will be worse environmental impact simply because you have to use more invasive means to support an aging population. More factory farms since there are fewer, older workers, more resource intensive automation and robotics to take over things people did.