>Don’t you think it’s likely more of a pendulum?
No. For it to be a pendulum, it would have to be true that some little girl, herself the only child from a long line of only-children, who grows up in a society where people have only one child or more likely no children, all her role models being childless... her school teachers, celebrities, everyone she's heard of, that little girl would grow up and say "I want to be a mommy and have 5/8/12 kids!" And that's so absurd I don't know why you'd even ask.
Nor is it likely, as many of those with unearned optimism believe, that some "high fertility families" can just outbreed everyone and come out the other side of this. Their children become indoctrinated faster than they can have them... the Duggars aren't on track for 400 (20x20) grandchildren. Insularity doesn't help, the Amish only survive with their lifestyle as long as our lifestyle exists... it requires too much technology that they buy from us. They either would need to change and become like us, or revert to technology from centuries before after which their fertility plummets.
>Maybe the equilibrium is 5-7 billion,
What's the life expectancy in China? Find that number, n. In that many years, China's population will be less than half what it is today. Much less. You have no intuitive sense of the math here. The current generation being born will live until that life expectancy number, then die (on average). So the current generation being born right now will evaporate in that number of years... but the generation they'll start giving birth to in about 25 years, it will be only half the size. The older generations, almost all of those will be dead in n years, since they're way past life expectancy.
It's fucking bleak.
>Even if we drop to 3 billion, the world would find ways to go on, likely implementing policies that make big families more favorable again.
There are no policies that can incentivize this that are also affordable. How much do you think you'd need to bribe someone who calls themselves "childfree" to have children? Sure, everyone has a price. Is that $1mil, or $1bil? And sure, the US government can afford even the $1bil number. But the United States can't afford $1bil times the 500,000 births we need next year. And if the US can't afford it, how the fuck will Portugal or Taiwan or Australia afford it?
It's not that you're thinking unclearly about this. It's that you haven't thought about it at all. Because no one's ever rang the alarm bell. And why haven't they? They keep telling you that it's no big deal. This is human extinction.