All Bitcoin-Hype aside, for easiness' sake, let's assume we're doing this with our own, special coin, let's call it RetirementCoin(RC).
In the beginning, people will need a lot of RC to pay into the pension fund. RC is new, its price is low, i.e., for a given amount of e.g. US Dollars, you'll have to buy a lot of RC and lock them in your address. Exchange rate of RC will go "to the moon"™, leading to lower amounts of RC required to put into your retirement address.
The years go by.
Finally, when pay-outs start, huge amounts of RC from the first pay-ins will pour into the market, pushing the exchange rate down.
Your second pay-out will be substantially less RC at a substantially lower exchange rate. You'll be practically broke once you retire.