It basically all comes down to tax policy. Boomers voted to cut taxes (especially on the wealthy) to roughly half what they should be, and ran up the national debt to match the GDP when there was no war or other crisis to justify it.
The founding fathers warned against that, because if one generation offloads its debt to the next, then it effectively enslaves the next against its will (deprives it of property, and taxes it without representation, among other things). This is self-evidently unsustainable, because any generation could do it at any time.
This concept can be trivially extended to all underfunded benefits. Maybe a shark raided retirement funds through legal smoke and mirrors but never got arrested. Maybe bankruptcy was abused multiple times to avoid paying workers, and that person not only didn't go to jail, but ended up a billionaire and president of the United States. You can see that this philosophy runs deep throughout our culture.
So I don't buy that pensions are underfunded because a company collapsed due to loss of market or whatever. It's more likely that the funds were raided the same way that congress has been writing IOUs for social security since the 80s. Somebody stole that money.
So I'd vote to pay down the national debt by raiding wealth created between 1980 and today. A high inheritance tax would do that. So would raising unearned income taxes (like capital gains) far above earned income tax.
Yes, these would slow the economy. But we're all cheating right now, living off the backs of future generations. Growth won't matter if gen X and millennials wake up and decide to throw a jubilee and cancel the national debt (my pie in the sky preference).