@ChrisMayLA6 Do you mean social or economic? There is utterly no plausible *social* downside to reduced or even zero birthrate.
If you mean economic in the sense of aging population depending on economic activities of younger people, then as long as the decline is only local not global, it's trivially solved by lifting limits on immigration and making immigration attractive. If a country refuses to do that, the problem is not birthrate decline but their shit immigration policy.