This pension-funding question comes up a lot, but I think it's a total distraction from the larger debate about the profitability of a public mail system. If we're debating whether, I dunno, a bake shop is a viable kind of business, the fact that the bake shop in question promised devastatingly large pensions to its employees decades ago is obviously an important fact about the state of that bake shop
at this actual moment, but it does not tell us much about the general viability of bake shops, which is the thing actually being argued about.
It's like asking if a family can live on $50k a year but then stipulating that oh by the way in this family the father gambles 90% of his earnings away at the casino on payday. It doesn't really answer the question.