Smaller birth cohorts have less political power in a democracy. So regardless of the absolute scale of human civilization, shrinking birth rates means the children who do get born have less power relative to their parents until their parents die.
For context, the "Baby Boomers" were only overtaken in population by "Millenials" two years ago. That's a very long time for a group of people to enjoy political power owing purely due to when they were born. This means that political establishments become gerontocratic, as there are simply more old people to vote themselves into power.
The population-equals-power dynamic also plays out on smaller scales, too. Countries with shrinking populations are at risk of being invaded or having their economies collapse. I'm not going to argue in favor of infinite growth[0], but I will point out that certain things we are accustomed to require a minimum amount of people on both the supply and demand sides of the equation. We can see this in rural America, which has been sucked dry of its population. The only people who still live in small towns are the people who are unwilling or unable to leave, and many towns have collapsed purely due to lack of people living in them.
[0] Though I do want to point out that I haven't seen a good argument as to why degrowth won't come commensurate with a massive surge in ethnonationalism. The only society I can think of that was wholly devoid of economic growth was pre-Meiji-era Japan, and that was the result of literally sealing the country off for centuries, banning an entire religion, and enforcing a very rigid political hierarchy.
For context, the "Baby Boomers" were only overtaken in population by "Millenials" two years ago. That's a very long time for a group of people to enjoy political power owing purely due to when they were born. This means that political establishments become gerontocratic, as there are simply more old people to vote themselves into power.
The population-equals-power dynamic also plays out on smaller scales, too. Countries with shrinking populations are at risk of being invaded or having their economies collapse. I'm not going to argue in favor of infinite growth[0], but I will point out that certain things we are accustomed to require a minimum amount of people on both the supply and demand sides of the equation. We can see this in rural America, which has been sucked dry of its population. The only people who still live in small towns are the people who are unwilling or unable to leave, and many towns have collapsed purely due to lack of people living in them.
[0] Though I do want to point out that I haven't seen a good argument as to why degrowth won't come commensurate with a massive surge in ethnonationalism. The only society I can think of that was wholly devoid of economic growth was pre-Meiji-era Japan, and that was the result of literally sealing the country off for centuries, banning an entire religion, and enforcing a very rigid political hierarchy.