One commonly perceived problem is the increase not just in the fraction of the population that is retired, but how long they will remain retired if provided with adequate care, or (more bluntly) how long it takes them to die compared to previous generations. Here in Germany the social system was established when most people died within two-three years of retirement age. Today they can expect to live for more than fifteen years, thanks in part to more expensive health care (cardiac arrest used to be effectively lethal, if not immediately then through subsequent arrests, now it's generally survivable with expensive care). The current retirees are entitled to this, having paid into the system, but current workers can look upon their own increasing contributions and the overall maths and feel uneasy that though paying more they may receive less.

The retirement age is now slowly being raised, and given time the system may level, but in the meantime "issues" will indeed present.

The issues might go away sooner than that, because I imagine - naively? - that a society full of old people would put lots of effort into reducing the problems of being old.

Having declined, the population might increase again, and decline again, but with fewer issues the more practice we have. Stability sounds unlikely.

> Why would stabilizing at a lower population be problematic?

1. There are no incentives to stabilize at a lower population. It is not like population are decreasing because of overpopulation.

2. It is workers / welfare recipients ratio that will be the catastrophic problem.

> Sure, during that decline there's going to be issues, but once you hit a new stable population those would go away.

I think you're compressing a lot of hardship into "issues"... It's not about total number of people, it's about demographic distribution. If half of your population is at retirement age, who is going to take care of them? Who will be working to generate tax revenue to provide the social services that you expect?

Also populations don't just stabilize, they feed back off of themselves. If half of your population isn't procreating, you get fewer and fewer babies which is a vicious cycle. There aren't many policy levers to pull during demographic free fall.

> > Somehow in the 1970s we got by with 1/2 as many people as we have now - why would a long, slow decline back down to that number be catastrophic?

Because we've built an economic system based on the idea that you must, at all costs, grow. Stable success is insufficient and will be seen as failure.

And then we spent many decades programming people to believe this is the only thing that can possible work.

Nothing natural about this at all, it is just the result of the cult of capitalism.