[TalnSG]

I think immigration would only be a temporary fix, at best.

The main factors leading to the low birth rate are not the lack of adults to produce children.

They are:
1 - Population density. New immigrant will make this worse and the rate could drop even further. It is a biological fact that in high density populations, the fertility of mammals decreases significantly.
2 - Economic. When commodities reach the point that it takes both parents working full time to provide enough income to raise children, then two things tend to happen. Women stop bearing children because of the stress of motherhood and a career (not impossible, but difficult) and couples tend to find distractions that make parenting less desirable.

Immigrants will not help either of these situations.