It would be wise to have some pro-natality policies here and there, but look at China what happens if you go all "existential threat" on this issue. Biology is not engineering, things evolve differently than what one wants (there are other examples of strong natality policies fails).
The power of collective action via votes isn’t a bayesian system, its just like the sum of many binary vectors.
Also, unless you throw the elderly to the wolves, having very low birth rates leads to a huge drag from having too many retirees to support with your shrinking working age population.
Not to mention automation is only increasing. Japan will have a booming robot export business going in a few decades.
A lot of these arguments about "needing" immigration to support aging populations are just thinly-veled neoliberal excuses.