I am saying that in a discussion about the risk to the 401k, mentioning social security is irrelevant.

If you've ever had to answer the question (or known someone who has to answer the question) "at what point do I begin to draw down from my 401k/IRA/whatever?", the questions "When am I eligible for SS? When do I receive the maximum SS benefit?" looms large. Social Security is an economic backstop for the elder that prevents a downturn in the market from forcing them to divest from their retirement positions at a disadvantage, precisely because it makes up such a large share of their retirement income.

If you are going to obliterate (or even just privatize) SS entirely, you're eliminating that counter-cyclical cushion. Now there is no baseline of income that elderly people can rely on. There is no guaranteed future income to borrow against, destroying the credit worthiness of the elderly. That's both immediately consequential during a downturn (market goes down, guaranteeing an equivalent drop in retiree spending) and historically consequential (apartments/retirement communities can't admit new elderly residents without assuming high risk of non-payment, credit agencies and mid-term lenders don't want to do business with the elderly, elderly residents are at increased risk of eviction/repo, etc) that make life for elderly people much harder.

that does not cover the growing gap the 401k needs to cover.

When the NASDAQ, led by the MAG7, has seen a historic 150% increase in a mere five years, it is extremely premature to talk about 401ks failing to cover basic needs due to a 15% downturn (off a post-inauguration spike). What kind of growth does a 401k need if the last five years isn't covering it? You're not describing a problem of market volatility, at that point. You're just saying 401ks suck as a vehicle for guaranteeing long term retirement needs.

And that brings us back to alternatives, like Social Security, that both already exist and are pivotal in propping up retirement income when 401ks fail.