The percentage of US workers with access to a retirement-benefits plan is 71%. 57% of all US workers participate in such a plan.

(Source: U.S. Bureau of Labor Statistics, 2009 National Compensation Survey, http://www.bls.gov/ncs/ebs/benefits/2009/ebbl0044.pdf)

In Table 2, it says that even 15% of the lowest income decile - people making less than 90% of workers - find a way to participate in a retirement plan.

Here's a more detailed writeup from the Bureau: http://www.bls.gov/opub/cwc/cm20100520ar01p1.htm

Meanwhile, the "top 10%" of households earn $135k per year or more, while the median income is around $50k.

(Source: http://www.economist.com/blogs/dailychart/2011/09/us-househo...)

Anyone making over $60k per year could (in theory; this is a highly abstract argument at this level, of course!) save 20% of their income without dropping below the median household income of $50k; households making over $60k per year appear to constitute 32% to 40% of the population, if I read this Census Bureau chart correctly: https://www.census.gov/compendia/statab/2012/tables/12s0690....

None of which says anything about the effectiveness of 401k plans, or anything else. But if you're going to handwave about numbers it's nice to hit the right order of magnitude.