Great analysis.
Demographics is destiny.
You didn't mention it but young people are far more effective soldiers than old people. So having very low fertility over a long period makes a country more vulnerable to invasion.

Russia had low fertility rates after 1989 too but during the Putin era they have risen significantly (but still below replacement).

Interestingly the other war in the news involves Israel, which has by far the highest fertility of any developed country (3.1) and Gaza which also has very high fertility.