Pension funds (and retirement savings in general) depend on low-risk investments returning significantly above inflation. These no longer exist, for a variety of reasons, demographics being one of many.
People have been assuming that negative interest rates are a temporary thing but people are beginning to think that this may actually be the natural state of affairs.
Historically, protecting your wealth cost real money: banks had to hire guards to protect the gold in their vault. Fractional reserve banking flipped this so that the banks started paying interest.
But now the world is awash in capital so low risk investments now return interest less than the inflation rate. Even the nominal rate is now sometimes below 0. (https://news.ycombinator.com/item?id=20696343)
There is no longer such a thing as a "safe investment". The only way to grow your money is to take risks with it, and the level of risk required to get returns significantly above inflation is growing.