Because the elders haven't extracted enough wealth from younger generations? Because economics has nothing to do with people choosing not to have kids? I'm picturing Germany as Sideshow Bob walking right into another rake.
I can easily understand that if everyone went my route (i.e. no kids) that society would collapse by definition, and my later years would be inherently miserable. I'm depending on others that do have kids (and sacrificed a lot in their 20s, 30s and 40s, a sacrifice I was not willing to make) so I can pay for medical and aged care when I'm old. So paying a slight amount more for this support seems highly reasonable to me.
Speaking as a German with children: Completely apt image. Yet I would name several dozen of policies that are more serious "rakes in your face" than this. This is merely squeezing a little bit more out of the working population. Everyone knows that social security in the current state is a Ponzi scheme and what ever is collected is immediately redistributed.