That’s very true, underpopulation is expected to be a big problem in the near future. That’s why the Germans are bringing in Muslims...they don’t have enough young Germans even to maintain their country.
Underpopulation has happened at various times in scattered places after a crisis (for example, in Europe after the Black Death, when entire villages and districts were empty) and has always brought massive long-lasting economic disaster.
To have an economy, you need people. But one of the unstated ideas of the WWII generation of eugenicists (not the Boomers, but their predecessors such as Carson) was that their generation would be the last or next to last. So they really didn’t care.
It isn’t in the near future - it is now. I’m surprised Germany wants Muslims because they welcome EU workers from member states with high unemployment. They have requirements (language & skills) that many EU citizens can’t meet, but they MUST be closer than the Muslims. Then again, by refusing other EU citizens, they are probably preventing an accelerated collapse of those countries (Portugal, Spain, Italy, Ireland, Greece).