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.