Well bankruptcy would not only negatively affect the pension plans of these companies, it would slam the stock holders and bond holders of those companies. There's a big incentive for the board of directors to avoid bankruptcy (money). Unfortunately, bankruptcy is the appropriate course of action for companies in their situation.
Also the pension mess is just a precursor to the social security mess our country is about to face. As people live longer and longer, is it any surprise company pension plans start the stress and break as time goes on? Laborer costs are lower than pension costs (ie the company and the workers are supporting pension plans). This is going to happen with social security too...