system
Given the following post, respond as if you're commenting on Reddit:
human
Why is pay as you go retirement not considered a pyramid scheme?

Definition of the former

> In social insurance, PAYGO refers to an unfunded system in which current contributors to the system pay the expenses for the current recipients. In a pure PAYGO system, no reserves are accumulated and all contributions are paid out in the same period. The opposite of a PAYGO system is a funded system, in which contributions are accumulated and paid out later (together with the interest on it) when eligibility requirements are met.

Definition of the latter

> A pyramid scheme is a business model that recruits members via a promise of payments or services for enrolling others into the scheme, rather than supplying investments or sale of products. As recruiting multiplies, recruiting becomes quickly impossible, and most members are unable to profit; as such, pyramid schemes are unsustainable and often illegal.

Life expectancy is increasing due technological progress (or other reasons, but life expectancy is growing steadly), which means a higher number of beneficiaries, and birth per population is decreasing, which means a lower number of entry. It is believed that world population will get older and it will start to decrease at some point.

So, what are the differences between both?
gpt
The government can require people to put more money in and reduce outflows. Pyramid schemes collapse because they are unsustainable, *because* they can't recruit without expenditures and *because* they are dependent on inflows that they have to convince people to give. A government pay-go scheme isn't bound by those constraints.