One definition of a Ponzi scheme:
'a form of fraud in which belief in the success of a nonexistent enterprise is fostered by the payment of quick returns to the first investors from money invested by later investors.'
The pensions are not based on nonexistent enterprises (though some of them no longer exist).
They do however, provide payouts to the early investors through the contributions from later investors. In this, they are like Ponzi schemes. They cannot pay earlier investors (retirees) solely off the investment that those investors put in. They require a constant influx of new investors (current working contributors) in order to pay benefits. Just like a Ponzi scheme, if the new influx of money stops or slows, the scheme fails.
If the pensions were truly sustainable, the average retiree would take out in benefits no more than what they payed in. If that were the case, retirees would never take one red cent from currently working contributors. It would not matter how many companies went under our even if no new workers ever got into the plan, you take out only what was put in in your name.
That's not how they were structured. In most pensions, the average retiree winds up pulling more than what was put in for them. They survived like a pyramid or Ponzi scheme, by the constant growth of the work force. Now that that growth rate is stalling, they are failing.
So now CS is trying to bone UPS by cutting benefits way more heavily to UPS retirees because UPS agreed back in 07 to make up shortfalls for those employees. Nice.
The current retirees, out of desperation, want a mortgage to get them into the grave at their current level, meaning in 30 years, the fund gets a multi billions payment liability and goes belly up boning you guys working now.
So basically, this proposed plan is a 30 year Ponzi scheme to extend the life of the old Ponzi scheme long enough for current retirees to shuffle off their mortal coils in comfort.