The growing population of economically non-productive people requires a growing population of economically productive people to support them. At some point, you could take 100% of the output of the productive people and it would still not be enough to support retirees et al.

At the limit, not growing the productive population puts younger generations in a position of existing solely for the purpose of serving the non-productive population. At some point, they will simply choose to opt out and the whole thing collapses.

I am not remotely worried about birth rates. Every tech executive hyperventilating about it is extrapolating social trends decades ahead, which is the same mistake Erlich made when he published the "The Population Bomb". The total fertility rate has limitations as a metric too (it assumes constant birth timing).

The fact that they do this coercive paternalism on the very platforms that substitute for real life social interaction is very rich to me. I'll listen to them when they divest from the social corrosion machines.

We need young people to pay for old people retirement (economically speaking, someone has to be working when someone else is just eating).

The main issue with population decline is the inability to depend on the growing younger population to fund the retirement of elderly people

Look at the problems South Korea is having, where there are not enough young people to support and care for the elderly. Elders face economic hardship and the healthcare system is buckling under load.

Because progress and growth makes us wealthier and happier? It's pretty simple.

People say "Oh, but GDP isn't everything" - but it's correlated with almost everything good, so might as well be.

If you planted an apple tree and it never produced apples, you might start to wonder what's wrong with the tree. Maybe there's something wrong with the soil?

How do you know if an organism is thriving in its environment? You count the offspring over generations.

humans are good. life is good. we should be trying to increase the number of conscious beings in the universe.

we have a diseased misanthropic culture. i dont know where it came from but its existential.

Because you are not prepared for the poverty that follows from an economy stalling.

Basically it makes people feel good. Growth is exciting and motivates people to do stuff. Shrinkage makes people sad, depressed and more likely to try to protect what they have. It's often irrational, but that's just the way it is.

Growth isn't sustainable, of course. If you're a gardener you get to experience the joy of growth every year, but you have to "pay it back" in autumn and winter as everything dies back and resets. The seasons force it on you in the garden, but we can't force it on ourselves. We'll just keep having summer after summer until it all goes boom.

Capitalist systems (even used in moderation by China) based on Keynesian economics relies on constant growth.

I think people really fail to understand the gravity of an inverted demographic pyramid, going from 2 young people supporting 1 old, to 1 young person supporting 2 old. That's .5 -> 2x, a 4x increase in burden (taxes / extra work).

The Social Security system relies on creating a debt of unborn children to older people based on those older people having already paid now dead people, so keeping it solvent requires more meat for the tax machine.

A pyramid inversion means the old keep voting for OPM from the young, using their numbers to crush them, meanwhile there are fewer and fewer young to actually pay it. Eventually creating instability, couple this with entitlement "I paid that dead guy, so that kid owes me!" (of course, abstracted, as "the government owes me" to hide the kinetics) and you are in a bad spot.

---------- edit: reply to below since I am throttled -----

yes under any system youth are needed. But SS creates a tragedy of the commons. Because retired get benefit obligation of children whether they have/adopt/foster the children or not. In most other systems, the link is more direct, so there is greater incentive to have or adopt child and provide investment in the child, as their success is directly linked to yours. In SS system you can reneg on most of the responsibility of creating the engines of the next generation but still simply scalp that investment off someone else, and indeed still get roughly the same share without making the investment. Obviously there is great moral hazard to simply scalp the benefit of children without having to make the investment yourself, and SS is all to happy to provide that.

American capitalists and economic planners fret about "Japan Syndrome". To have more productivity and more consumption i.e. GDP growth, you need more people as a core driver. We don't actually need this, we could do fine with a stable population, but capitalism needs to grow or perish.

Declining populations are trickier for most economic concepts though. Less labor, less consumption. That said, a slight decline can leave more houses unoccupied which can be good. A major decline would mean so many unoccupied houses that you would have broken and abandoned houses though because it would be too costly to deal with the abandoned units.

The line has to go up every year forever, even if it causes cyclical market instability and consolidation into mega conglomerates. Creating sustainable wealth across all sectors of society just isn't profitable enough in the short term.