Congress did that to the Post Office and it nearly killed them. Worse, it made them less competitive with FedEx, UPS, DHL, etc... which further exacerbated the problem.

You also see this with big legacy companies that slowly get throttled by their pension schemes and are undercut but new companies full of young workers. Automakers for example. We have been underfunding pensions for a long long time because people weren't supposed to be living so long and because cuts to the pension program aren't immediately visible so they're a tempting target for unscrupulous management.