In the US at least, almost half of elderly would be living in poverty but for social security. Most are not retiring and living off of investment income.
Investment income depends on a functioning, growing economy.
Even just maintaining stored retirement wealth depends on this. Think about prices for services when there are large numbers of rich retired people with needs, but few young people willing to provide them.
The answer, of course, is automation. Granted, a ton of thing may just not be possible with robots. But I expect that, just like Oil was the resource that boosted humanity, AI (and modern medicine) will be the resource that comes to aid the elderly as demographics rapidly change.
It does not depend on a functioning, growing economy at all. TIPS and Series I savings bonds require the U.S. Govt to not default on its obligations. They preserve purchasing power.
This is news to those who retire and live on investment income.