There are a number of ways that can not be true without a large scale societal collapse. Fraud involving pension funds has happened many times in the past (Bernie Madoff, Robert Maxwell being two high profile examples). The last financial crisis brought a bit more attention to the topic of counterparty risk - the idea that your "safe" investment is only as safe as the institutions that are backing it in many cases. It's not necessarily a high priority concern but I think it's worth at least considering splitting your retirement savings across more than one account with different institutions.
There are also lots of conceivable larger scale crises with historical precedent (many in the 20th Century) that would render your retirement savings largely worthless without leaving you dead. In many of those you would have more pressing concerns than your 401K but it still seems like not a bad idea to have some physical things of value that you keep somewhere secure but accessible (cash, perhaps gold and/or silver).
These concerns are all likely fairly low probability but there's certainly a whole range of possible scenarios between "my retirement funds are completely secure" and "I'm dead in a global thermonuclear apocalypse".