I started my post with "People are saying" in order to stay rather neutral on this issue. I'm not arguing that it is true that we need to have more kids, just that if it is true, having 2 kids isn't enough.

Our infrastructure and institutions were built under an assumption of continued population growth. Old people need to be taken care of, and under population decline there aren't enough new young people to do so. Infrastructure built for a certain population size becomes unmaintainable if the working population size gets too low compared to the elderly population size. And the burden on the working population becomes so great that many of them leave to other countries, causing a vicious cycle. Societal collapse accelerates and is unstoppable.

Japan has this problem right now. There are tons of elderly, and the working people work every single waking hour, sleep on the train, beat themselves to death, under a sense of duty to their society.

Robots might help.