> The whole tone of discussions about low birth rates always contains this implicit assumption that there's anything wrong with it
First, only certain societies have only recently arrived at a value that there is nothing wrong with having more kids. Traditionally, having babies and raising them was considered to be of utmost importance.
Second, it still is of utmost importance given how our societies are structured. You want to work until your 60's, then retire and live until mid-80's? Fine. But who is going to work/pay for your food, entertainment, healthcare, security and general societal infrastructure? All those things need humans which you are not going to raise.
So "developed" societies seem to convince themselves that there is nothing wrong with not having babies, while at the same time utterly reliant on "underdeveloped" societies to import people from.
> investing more resources in fewer kids is what pays off
How do you plan to balance Social Security and budget with fewer people working?