Because most pensions are tied up in the market. So if the banking system deflates and resets, it drops the market value of pensions significantly.
The market essentially is a herd animal. Everything goes up or down together, in aggregate. Since wisdom in the financial market is to diversify, ie, hold in aggregate, a well-chosen portfolio generally does about as well as the market in the long term.
So if Grandma had 100K in her diversified portfolio of low-risk investments and the market exploded 50%, she'd have 50K, which represents a major loss of financial capabilities.
Whether it is better to drop such people in the cold while things reset or whether it is better to keep the cushion going, I leave to another media form, as such discussions tend to be too inflammatory online.