Why Should We Worry About Declining Birth Rates?

We don't want to reverse the causes of declining birth rates.

    1) Car culture (and trespassing culture) eradicated free roaming
     areas for kids. This resulted in
    2) parenting time is 20x higher than a few generations ago
     because
    3) children live in adult-curated boxes, under 24/7 adulting 
     because
    4) they have no where to go. See 1.

In the US, a depopulated civilization would be allowed before we abandoned ~mandated driving.

I think people take the idea that birth rates will stay low for granted. It’s entirely reasonable as a hypothesis that people are simply responding to environmental pressures against having children, and that these pressures will subside as the population shrinks. Not factoring in technological change, I think we’re just responding to our planet’s comfortable carrying capacity without really realizing it. This makes me think we’ll eventually see birthrates stabilize as space and resources become less scarce to future generations.

The arguments for why we want the birthrate to be higher seem to be: 1. More people, more "ideas" 2. There are things that need to be done that have fixed costs, more people, cheaper per person 3. Basically, more people is better because human life is good

The second to last huge population decline was the Black Death. The loss of population made labor scarce, frayed the knot the feudal system had on people, eventually resulting in increased mercantilisme, urbanisation and ultimately The Renaissance.

The first, was of course, The Great Dying of the Americas. So things really could go either way.