There are a lot of lazy scare tactics that pension opponents use. The parent commenter's use of "pyramid scheme" means that he or she is trotting out some variation of the argument that current benefits for retirees are paid by current employees, that in 1940 there were 30 workers for every one retiree but it'll someday be 2:1, etc. etc.

"Productivity growth" is my admittedly lazy shorthand for "You cannot ignore the effects that improved productivity has on pensions, especially when by necessity we're talking decade-or-century long timeframes. Even historically modest productivity growth means that if a worker is supporting 1 retiree this year, then next year he can support 1.015 retirees and in 60 years the average worker can support 2.4 retirees. Without mentioning why you (the parent commenter) think that productivity won't continue to grow at at least a very low level over the next several decades, then we can't really have a good discussion about pensions being pyramid schemes."

And that's before we even touch on other topics like the retiree population shrinking as baby boomers start dying over the next 30 years, the double-standard of treating purchases of U.S. Treasury bills by current workers (for U.S. Social Security) as different than, say, a hedge fund buying them, etc.

Bottom line is that pensions are complicated and can't be treated simply as if they were just a regular investment fund or savings account, and it's a waste of time to argue with somebody who just wants to handwave away the differences.

OK, so productivity increases and in 100 years 1 worker's production can support 100 retirees. How? They create stuff using natural resources. That stuff gets sold, who is buying it now that 99/100 people are out of work? How are those 99/100 going to pay in to a pension?

Oh right of course, new areas of industry develop so now all 100 people are still employed and they're producing enough stuff for 10,000 people. But that means they have to also be consuming 100 times what they did. Where is the energy coming from, where are the resources coming from to make this stuff?

Isn't it really the case that improved productivity gets to benefit the wealthy disproportionately - suppose in 60 years a worker can produce 2.4 times more output and thus support 2.4 times more retirees. That's not how it works financially, the worker gets paid maybe enough to support 1.5 times more [through taxation] and the other output increase goes to benefit the wealthy capitalist. Regardless population has tripled in the last 60 years [Wikipedia figures].

In practice state pensions in my country [UK] are reduced year on year and recently pension ages have been increased.

It sounds like you're banking on a sudden reversal of global population growth (the best case scenario of UN statistics shows continued growth for at least a couple more decades; their "best guess" is continued growth until at least 2100).