Outcomes aren't great for old folks who go into homes without having someone on the outside looking out for them. I don't know if you've seen the recent stories but a little cognitive issues and you'll be signing away everything you've earned to the home itself or one of it's employees. It's quite common.

Having kids solely because you want someone to look after you when you're older seems like an extremely selfish reason to have kids

"Solely" is doing a lot of work there and making a great deal of assumptions.

Yeah but that was the whole point. You're telling someone who already doesn't want kids why they should want kids, but you're giving them only one reason. If they do so, it will only be for that reason

Not necessarily. Suppose that person has multiple reasons to want kids and multiple reasons to not want kids. They analyze the pros and cons and figure that the cons slightly outweigh the pros so they choose not to have children.

If later they learn something unpleasant about nursing homes, or pension plans, or something else relevant to their expected quality of life at their old age, then that might tip the scale towards having children, even though it is only one of many reasons.

The last reason that tipped the scales towards a decision is not necessarily the only one or even the most important.

Why would I care what happens when my IQ degrades to one below room temperature?

Similarly, if I go into vegetative state I hope someone will use my organs for some good cause, not try to keep me alive by any way possible.

Unless you’re investing in canned soup and ammo, you need young people working for those retirement “savings” to be worth anything.

The modern retirement system is merely an additional layer of indirection over the traditional system of kids taking care of their elderly parents.

Yeah, if the number of people shrinks, so does the size of a market, and so do revenues of companies. If you own stocks, those will be worth less. If the number of people shrinks, so does the demand for living space, and the value of your apartment goes down. Etc.

Young, productive workers and consumers will exist somewhere globally, it’s a matter of capturing some of that productivity as investment return. If it all goes to hell, sure, the rows in a database are going to mean very little and you’re going to fallback to fuel, food, and firearms until the end (unlikely).

Not all markets are equally accessible by public companies as is the case with the US. Often you have a lot of private companies, companies owned by people close to the government, etc. Also, in coming decades, declining birth rates will be a global concern, not just one of the developed countries plus russia (and OMG does russia have this problem right now).

And hope when you age out, you keep your mental faculty strong enough you don’t get taken advantage of.

Or just don't be afraid of getting old. We all gotta die at some point. It only takes a week to starve and I can handle that fine.

There will be a lot of poor suckers. A lot of pension schemes and even social security in the US rely on having younger workers paying into it. It's a huge part of how people do retirement planning.

If your answer is that you are not planning on counting on it for the future then a lot of people will ask why not stop paying into it and keep the money taxed for yourself? It is a significant tax. You CAN actually stop paying into it and then make yourself ineligible for payments out which includes spousal benefits being cancelled. You CAN conscientiously object, it is a provision of the SSA law.

If a lot of young people saw that, they would withdraw from it and put the trust right now into a deficit which would adversely affect the people withdrawing from it right now.

I think there is a non zero probability of this happening regardless for different, uglier political reasons but it doesn't make a lot of sense for people to be forced to pay into something with no promise of ever getting something back.