1. It is a PROJECTION, based on a lot of assumptions and based on plan balances and market values as of a certain date.
2. Unless you are planning to retire in the next 2 to 3 years, it would be foolish for you to make a lot of plans based on projections, since the market values and other factors upon which the projections are based will certainly change over the next 5 to 20 years. (And if you are planning to retire that soon then NONE of the proposals out there will affect you in any way.
3. It sounds like you are comparing dissimilar items anyway, and perhaps what you are calling a "pension" is some type of savings plan like a 401K for example. Pensions are usually defined benefit plans which would not vary to the extent you are describing. A 401K plan could see a substantial drop like you are describing if it is heavily invested in volatile equity funds, but again the value/balance of the plan accounts today is not a reliable predictor of what the balances and thus the monthly payout of the plan will be in 15 to 20 years.
Let me offer an evaluation of your "real-life" anecdotal example of private pensions versus social security as it relates to the debate over social security reform: In a discussion on strategies for ensuring a long-lasting marriage, somebody points to the hillary rodham and bill clinton marriage and concludes that serial philandering and procuring bl*wjobs from adoring sycophants with the assistance of government employees seems to improve the health of a marriage. The "experience" seems to support the notion, but it is far from evidence that the conclusion is a universal truth.
Exactly macaroona's point. If she goes to retire and her pension is worth zero because some radical muslim just dropped a nuke on Wallstreet, all this "put your money in the market for longterm growth" becomes a lot of hooey. I haven't seen anyone suggest that 10% of your assets should be in hard currency, like gold and silver, and in your safe, just in case of the above scenario.