How can a shortfall happen, technically? I mean, why not just print enough money to inflate it, to cut real values of pensions to make ends of the system meet? After all pensioners aren't likely to riot.
The real problem with these pension promises isn't just that we're promising money we don't actually have... the problem is that we're promising a quality of life we can't sustain. We can't give every retiree a life that matches their pre-retiree life in most material ways (perhaps minus a mortgage), and promise them all the best health care that they could possibly want, and promise them that they can retire into this at 65 and keep all these benefits no matter how long their life span may improve, and that no matter how the social winds may blow in the future or how the economic winds may blow in the future that these benefits are written in stone. This ultimately isn't about the money, it's about what they expect this money to be buying them. No amount of accounting tricks can fix these problems for real; focusing too much on "dollars" obscures this fact.
The entire concept of fixed benefits in the future is a lie. It was always a lie, even if some people have managed to cash some in. The future is not predictable enough even before we consider people trying to game the predictions themselves or changing their behavior in light of the predictions.