There's one other possibility--more research into aging. I suspect that quite a bit can be done to keep people healthy longer. The first promising bit has happened already. Tweaking mice genes so that they have more anti-oxidant in their mitochnondria makes them live 20% longer, and it seems to include retarding aging. This is a long way from something you can buy at the drugstore but it's pointing in the right direction.
Letting immigrants in is the shorter-term, more certain path, but it's going to wear thin if current trends continue. Everyone's birthrates are dropping. All it seems to take is education for women and access to birth control.
In any case, I agree that we should let more people be legal immigrants, though it's more on moral and logical grounds. The logic is that I hate hearing, "Oooh, those awful people, it costs so much to have them around. Let's deny them as many economic opportunities as possible so they won't be able to afford to pay for much." The moral side is partly that I don't see that A has the right to tell B that they may not employ or rent to C and partly that it offends me that restricted immigration makes atrocities and slavery much easier. Afaik, illegal immigrants are the only source of slaves in developed countries. And it's almost amusing that the "family values" party supports adding a severe risk of death to people who want to both visit and support their families. Of course, so does the "compassion" party.
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Getting back to your article, they already have raised the retirement age. Not much, but it's 67 if you're born after 1960.
Illegal immigrant buy stuff, too, though less than if they were better paid. Perhaps more interestingly, their social security payments can be ripped off. (There may be something in the Bible against stealing from poor people, or possibly against stealing in general. I suspect there's nothing about illegal immigrants being an imposition that it's ok to mistreat.) Many employers try to make illegal employees fit into the record-keeping requirements, and that means withholding SS payments. Keep an eye out for the lawsuits when illegal immigrants have enough power/respectibility to try to collect that money. Meanwhile, I've heard that illegal immigrant payments cover about 10% of the shortfall.
I see that I let my libertarian principles slip--I'm not addressing whether Social Security is a good idea in the first place, nor whether insufficient demand is actually a problem. As for the latter, I have been mulling the question of how a free market economy would react to people basically having enough stuff, and whether it's really a problem.
We also get the question of what "enough stuff" means--I suspect we're getting closer to that point, and there's going to be more and cheaper stuff in the future. Not only will manufacturing get cheaper, but ebay has made selling used stuff more efficient. At this point, we're seeing many of the babyboomer's parents houses get broken up, and the babyboomers possibly even larger stashes are going to be heading towards the market (or the dumpster) in the forseeable future.