Exactly. Pension systems where generation i+1 pays for generation i are fundamentally broken. If the population size sinks, generation i+1 will pay more. If paying more and thus having less disposable income makes them defer having babies, you have your bad feedback loop right there.
A better and stable system is where generation i pays for generation i.
Most rich countries pension systems depend on sufficient workers to pay for pensioners.
How many rich countries are running a surplus this year in a non-recession year.
In 20 or 30 years with far fewer income earners per pensioner how will it look.