[SirMaster]

But surely something would change partway through those 25 generations?

Why would anyone assume that just because we enter a decline that we would remain on that same trajectory?

Surely when there is enough decline, resources become plentiful for those that remain, and people have more kids because of that? Just my naive thought.

[RandomCitizen12]

Well, one of the things that's changing is the creation of pro-natalism movements....

But there is reason to expect the trajectory to change. Currently the expectation is that, even if most people died out, there are very fecund (and very religious) groups that would repopulate the world. So the pro-natalism movement isn't needed to save humanity, just to save the 'normal' people.

[big-green-man]

The problem is that such a decline has intense inertia. When you have declining birth rates, average ages go up. Caring for elderly becomes a more significant chunk of the budget of a society. An only child for example has to care for 2 parents, potentially grandparents, themselves, and if they have a kid, half a kid. When you socialize this, people who have kids subsidize people who don't.

You can see how this would spiral. The society that makes it out of such a spiral is unfortunately the society that stops caring for it's elderly. The transition period would obviously be extremely unsavory and socially catastrophic. What winds up happening is people become incentivized to have kids so that they have someone to take care of them, and those who have more kids inherit the society a couple of generations down. That's what reversal of this trend looks like, it's not just about population numbers, median age is an even more consequential factor.

[SpicyLemonZest]

I don’t see why we would expect plentiful resources to lead to more kids. People today have fewer kids than they did 100 years ago, despite having many more material resources.

I’m also not sure that population decline would lead to plentiful resources in a sense that matters. Do people in Detroit feel rich because houses are so plentiful after the 1950s population high?

[from-nibly]

it's because now having kids means sharing your resources, where 100 years ago having more kids meant expanding your resources.

[big-green-man]

So we have turned an asset into a liability and are surprised when the change in incentive has predictable outcomes.

[c-linkage]

The key element of making dire predictions is that you can't assume anything will change as a result of publishing that prediction. Otherwise your prediction will no longer be dire.