There are other investment vehicles besides equity and other ways to take value from past into future that are neither garnishing the wages of children and grandchildren (most current state systems) nor investing in artificially inflated markets.
Just as a thought experiment, consider if all state pension contributions were just used to immediately purchase gold on the open market that was then put into a vault labelled with the year of birth of the contributor. Please explain how this (obviously naive) strategy is dependent on future labor. As far as I can tell, this system would be completely market-based and future labor would likely benefit as their “gold” might be cheaper as there would be less demographic demand.