My view on it is:

The world turned before we existed, it will keep turning long after we're gone.

The economy is manmade. It was modelled for humans by humans, and if we have to change how it works for it to survive, we will always find some bullshit new rule to append to the rest.

Making kids to sustain an economy is just a pyramid scheme. Don't make kids other than because you want them and you want to love them otherwise you're just scamming them.

That's an easy one - no. You can look back to various periods during middle ages Europe for examples. An even stronger one would be China from about 400 CE-800 CE

Of course, those weren't capitalist economies - but they were economies. Capitalism's instability is what requires constant growth to maintain. The better (and harder) questions would be what to transition to that avoids the issues of feudalism and how to transition with a minimum of societal upheaval (violence and death).

"pay back the debts" of previous generations

It is pretty important to certain capital-holding, debt holding folks that we only talk about "paying back" debts, and ignore the other obvious answer:

Not paying back the debt.

Eventually, when a debt system becomes too burdensome, people respond to it by...not paying.

But avoiding talking about "not paying" is pretty important to anyone who wants to make the system maximally painful, short of not paying.

Naming "not paying" risks causing people to think about "not paying".

It's far more complicated than this, but it's worth staying aware that "not paying" is the real release valve, and most expert analysts are being paid extra to not talk about the topic of "not paying".

but also ensure that the population keeps growing in order to produce more and more human labor to "pay back the debts" of previous generations, because all money is borrowed from somewhere else...

This seems like an issue with accepting the current economic system as the only system or leading to some neo-feudalism.

We could restructure society and the economy so that the "debts of previous generations" weren't such a fucking weight.

Infinite growth is referred to as cancer. Your friend is obviously right that we cannot sustain infinite growth, but it's misguided to think that the only way out species can possibly survive to any length is by having more children and increasing our population year over year.

With improving technologies and automations, far less labour is required to achieve the same results. There is no reason we need an infinitely increasing population on our decidedly finite earth just to keep our species afloat. This would take a major restructuring of our social and economic systems to do correctly, otherwise we run the risk of centralized wealth mucking it all up, but the point remains that there's no necessity to continue reproducing at the rate we have been. This supposed "need" for labour is just capitalist propaganda perpetuating the idea that work is inherently good, all designed to fuel an inherently exploitative economy. Line must go up, otherwise how can the privledged few assure that their net worth continues to grow exponentially?

more and more human labor to “pay back the debts” of previous generations

There is no law of economics stating that a generation of people has to consume more than it produces.

No.

To preface I think reproduction is a very personal choice and right that we shouldn't force on or take away from anyone ever.

But let me ask you this: the planet we live on is finite. It's not getting bigger and its resources aren't getting more. Is infinite growth of population (or anything else, for that matter) possible here? Or will this system eventually collapse?

In terms of overall resources, it is probably better that we don't expand the population.

In terms of economics, we are finding more that, while people are living longer, they aren't living at the same quality of life, which means their economic utility drops significantly while still requiring an income. The current funding of retirement won't work going forward. Taxes from the wealthy could pay for it, especially as productivity keeps increasing and the rich seem to gain the most. But, there is going to be a push and shove against it as retirement of that many people effectively becomes an age based universal basic income.

Why do they think population is proportional to ability to "pay back" debts? We have technology. If one example of a "debt" is taking care of the aging baby boomer generation, yeah there was a time when that would have been solely the responsibility of their descendents, but we have improved medical technology to keep old people healthier in life, we have conveniences that make getting groceries, doing activities, or socializing easier, and (in some countries) we have modern social safety nets to ensure that even someone without any living relatives can feel safe knowing that they are taken care of.

Another way to phrase my original question: would it be adequate for us to, instead of increasing the population, to develop a series of sufficiently advanced and efficient robots to do whatever task your friend thinks we need more humans in order to do? Just trying to understand the rationale.

The economic system is broken. It incentivises infinite growth in a finite environment. That's just not sustainable and will backfire in one way or another.

What we need as a civilization is a shift in economic values that would push for a more sustainable model. That shift can be seen, but, in my (expert /s) opinion, it's too little and too late to completely avoid a disaster.

1. So in the beginning there were people. And those people had problems (need for food, shelter,...).
2. So to fix these problems more efficiently (but far from completely), people invented economies.
3. But now these economies require infinite population growth (i doubt that's actually true).
4. Therefore people make more people to sustain the economy. These new people themselves have needs, which did not exist before.
5. So these new people now also need to make even more people to sustain the economy.
6. Repeat.

So the economy actually amplifies suffering (exponentially), not reduce it.

Also, like others have pointed out: Many economies before industrial agriculture had basically 0 growth, and sustained themselves for centuries.

In our current economic system? Absolutely, declining population is a huge problem.

As far as physics? The world doesn't care about imaginary human numbers. Production continues to soar through the roof

We made all of this up. At any point, we could say "hey, this is a dumb game that's making people suffer, let's figure out something else"

I'm not sure what we're referring to when we say pay back the debts, but as far as a societal collapse due to constant population growth - is that not just the basis of nature? If resources become scarce, population will decline since you can't provide for everyone's needs at that point. If resources are plenty, population can grow. I don't see how a decline in population leads to societal collapse. Some countries are facing declining populations now, for other reasons, and they're not collapsing. It's a potential problem that would need to be faced, but collapse seems extreme to me.

Is your friend a Cold-War era Romanian dictator?

No, you cannot keep the population infinitely growing just to maintain the pyramid the economy is built on. I think greed clouds our vision, it would and will be possible with automation and AI to have a rich and technologically evolving economy with fewer people, the problem is people seem to feel they need to exploit other people or it's not a reasonable economy.

Statistically 2.1 births/woman is required to replace the current population.

As for the economic argument, your friend is somewhat correct, except that economies don't just grow or shrink based on population (it is a major driver). There are too many factors at play to make such a statement.

The finite earth argument is interesting, whilst we are the biggest danger to the biosphere in the short run, we are also the biggest hope. In the long run the biosphere will sort itself out after we are out of the picture.
Taking this argument a little further, we may be the only hope for an intelligent civilisation from this planet. We have taken all of the easy energy resources; which take millions of years to regenerate; so any intelligent civilisation that follows after us will not have the luxury of cheap abundant energy.
So we either sort our shit out, become space faring, and move on with the next phase of the human experiment, or the likelihood of intelligence leaving earth is quite low.

We could, reduce ourselves content to "save" the earth and exist here in perpetuity, but I don't really see that happening. There will always be those that dream and strive, if humans still exist in 10,000 years they will be spacefaring.

We don’t need infinite population growth to support the economy but eventually we are going to hit a point where we have to support more retired people than workers. At that point the economy will slow down as there is less demand because people will lose their jobs because of the less demand. People will also be spending more of their time and incomes taking care of their older relatives.

Our current Ponzi scheme, robbing Peter to pay Paul, method of funding social security will break down at that point and we will have to decide whether we send mom and dad and grandpa and grandma to the poor house, increase taxes on the workers who still have their jobs, or redistribute wealth from the 1%.

People won’t vote for the first 2, so we might actually get socialism at that point but it will hurt a lot on the way there.

Japan's population has been in decline since about 2005 and they haven't exploded in a fireball yet.

I think neo nataliam started as a very extreme view. I didn't think it was quite as sarcastic as A Modest Proposal, but it has the same vibe to me. I'd be suspicious of anyone talking it seriously at face value. Maybe they aren't being malicious, but they're at least unsophisticated.

This video would then be quite releveant

Not that I am a huge fan of him in general but you should watch this. Also there is no debt that's not imaginary. https://www.youtube.com/watch?v=fTznEIZRkLg&t=28 It is not strange that relatively rich people have fewer child than poor people. Rich people don't need a buffer for kids dying and being providers at your old age. The answer to population growth is obvious and anyone arguin anything else is speaking in their own interest. Equality in lifestandard, money and life.

To my current understanding (especially of whatever economy we speak of), that depends on if there's hyperinflation (have less children?), hypershrinkflation or whatever it's called (pump out those wee babs?), or hyperstagflation (akin to the first thing?). In a world where the ratio between people and money is equal to having less, I never understood why there is no "human standard/factor" in the "gold standard" sense, though then again these are the same people who insist on unchecked social incentive.