Agreed
> Only if the elderly continue to consume pensions.
Well no, unless you literally starve them to death. State pensions or money in general is just a redistribution mechanism which doesn't change average available resources. What really matters on a higher level is the fact that a big part of population (elderly and youth) consumes but doesn't produce. Whatever they consume is either directly produced by the working part of the population or comes from the foreign exchange of the same said produce.
So if the state pensions system collapses it just means that instead of feeding your parents through taxes you now do it directly, which on average is almost the same (difference coming from elderly who don't have kids to feed them). Which still means you have less resources to spend on your kids.
The way this is currently playing out in the UK, is that pension ages are increasing.
At the extreme case, there's no more state pension: if you don't have a private pension, you're working until just before you die.
I was thinking more of heating (and, increasingly, cooling) rather than starvation: Food specifically isn't the main concern here, unlike in the 1800s, because of how few farmers we need to feed the entire population.
(And this whole thing may be inverted at short notice in a whole bunch of different ways; UBI is basically a pension age of zero, but I'm only expecting that when AI gets good enough, and IMO that's "at least five years after self driving cars, whenever that is".
If it had been 2.4 children per woman* over that time frame, the current world population would have been 200e6 * (2.4/2)^(1000/25) = about 300 billion in 1000 AD, and 200e6 * (2.4/2)^(2024/25) = 514 trillion today.
> elderly don't only consume resources but also directly compete for them with the youth (the other class of people who consume without producing) which makes the fertility economics even worse.
Only if the elderly continue to consume pensions. If the economy collapses, pensioners protesting or voting for the other party in a democracy stops being an interesting threat, for the same reason they weren't much of a threat in 1889 when people were first taking ideas like "state pension" (in their modern usage and not e.g. medieval almshouses, and universal not e.g. what the Romans did for the military) seriously.
* I know I didn't mention "surviving to reproductive age" before, but that distinction is more of relevance to historical comparisons than future ones. I have no reason to think infant mortality will go up, after all.