That there will be nobody to care for the great mass of old people, nobody to work jobs to make 401ks worth anything, nobody to prop up social security, nobody to buy old people’s houses so they can use that money to pay for nursing homes So… all the old people will suddenly find they can’t retire and are forced to work till death. They will consider it a crisis. The current retirees are probably the final retirees in our lifetime.

But wasn't the idea that technology increased our productivity so that with few people we could do the work of many?

Almost all the productivity gain got reaped by the 0.1%

If the burden is too high, there's a risk that future generations will simply decline to care for the elderly who contributed to the problem (childlessness), but then expect the children to care for them in retirement.

That's almost a "positive" scenario in my view, I would prefer people who are left to care about themselves and their kids rather than the elderly who screwed them up.

The alternative is, more and more resources will be allocated towards the elderly and by consequence less towards kids, making the problem worse with every generation. Not clear though how to break out of this spiral by democratic means if elderly are the majority.

Except, the elderly aren’t the majority. There are just as many Millenials as Boomers. It’s a matter of getting Gen x, millenials, Gen z and Gen alpha to actually vote.

Democracy is little more than simulated warfare. Rather than harsh campaigns and brutal battles over every issue, society has learned it's cheaper to predict the outcome and avoid the bother.

But if the majority voting bloc is geriatric, then that does rather change the fundamentals.

Honestly self-inflicted economic problems seem mild compared to overpopulation and overconsumption. I am sure there is a happy steady state somewhere but I don't think we have the collective will to find it.

The fertility decreases are very steep.

Are we headed toward negative population or something?

In some places, yes. South Korea is expected to shrink by 15M in the next 50 years, and to cut in half by 2100. Even with immediate drastic improvements in birth rate, it is expected to shrink significantly.

Don't populations typically oscillate sinusoidally?

What's that got to do with whether such steep declines are desirable?

Well that might impact the long term projections.

The frequency/period of the sinusoidal wave you're asking about can be very long. When the Roman Empire collapsed its collapse was in great part due to Rome's inability to reverse a steep, long, secular fertility decline. It took hundreds of years for Rome to go from growth to decline, then hundreds of years for the post-Roman Europe to get back to growth.

When a generation's female cohort is significantly smaller than the preceding generation's female cohort the increase in fertility needed to offset that is enormous relative to the fertility rate that led to the present generation's small female cohort. And the cultural and other reasons for the low fertility rate are difficult to change too. So the likelihood of reversal is very low, and the likelihood of continued female cohort decreases is high, therefore you have to think a severe low fertility patch will last at least two generations, and if the fertility rate right now is half the replacement rate then that bakes in a population reduction of 50% once the current non-fertile generations pass, so several generations down the line. But the risk of three or four generations of below-replacement fertility is pretty high, and that's how you get into multi-centennial periods. Nothing requires that we bounce back, either. We could go extinct just from refusal to reproduce.

Yes but the peaks of those oscillations can be extremely unpleasant.

Like everyone suddenly gets raped/killed while all the buildings are burnt down kinds of unpleasant.

Negative population, no. The minimum is zero.

Population declines, absolutely -- those are baked into the pie now, and they will be quite steep.