I won't be adding much but want to make a small correction: it makes any retirement system difficult. The non-European-style system in the US, where people just keep stocks in their 401k and IRA accounts, is equally in danger when the number of people shrinks, since the returns from those investments depend on the growth of the economy, the size of which in turn depends on the number of people.
I realize you addressed this in your second paragraph, so as I said this comment isn't adding much, but I think it's worth spelling out: anyone who wants to have a dream of retiring before they die, better pray that the "overpopulation crisis" actually materializes as severely as possible.
No, you're totally right. The whole "Social Security is a Ponzi scheme" critique misses a basic point, which is that all systems of retirement are Ponzi schemes, or more charitably, systems built on paying it forward. The basic human life cycle, where the laboring generation takes care of children and the elderly, has not changed--we have just decoupled the process a bit using financial instruments and government programs.
When people "save for retirement" they put money in a 401k, they don't fill a cellar with creamed corn and MREs. Those investments are nothing more than earmarks on future production. The value of a stock is ultimately built on the right to receive a share of profits from the labor of the working generation. The same is true for bonds and other financial instruments.
No matter how you structure the retirement system, the physical reality is that the retired generation gets a cut of the goods and services produced by the working generation. As the latter gets smaller relative to the former, you have problems.
I realize you addressed this in your second paragraph, so as I said this comment isn't adding much, but I think it's worth spelling out: anyone who wants to have a dream of retiring before they die, better pray that the "overpopulation crisis" actually materializes as severely as possible.