Any given increase in average investment yields translates into a much smaller percentage increase in total income for most workers with 401Ks than it provides to people whose primary income stream is investment income.

You mean like people who saved in their 401(k) for 30 years and are now living off it?

People who are living exclusively on 401k income are, by far, a minority of people with 401k’s, yes.

why does it have to be exclusively 401k?

Because people whose support comes from other sources have the income-boosting effects of anything that bumps investment yields reduced in proportion to their non-investment sources of income.

Yes, a lot of (especially the upper income segment) of the working class has some investments, particularly in retirement accounts; this doesn't mean they generally see the same income benefit as major capitalists from policies that boost investment yield.