Does the law of the land apply to the whole world?
Population is not declining in the whole world. Africa, South America and the Far East minus China continue to see growing populations.
Reduced infant mortality due to improved health decreases the need for births to replace deceased children. This does not explain decreasing populations due to abortion, contraception, homosexuality and life styles that do not value parenthood.
I cannot imagine what kinds of rationale you are looking for to excuse the effectiveness of the anti family crowd.