The 401k experiment is not failing because people don't understand how interest and capital gains work. It's failing because people are not funding their 401Ks and the ones who do, don't sufficiently or consistently fund them. I know so many people my age (mid 40s) who don't even save at all. Like zero. They are playing a dangerous game of chicken where they believe the USA ultimately won't let the elderly die en masse in the street, and they think they'll be rescued. They think I'm the sucker for saving the max every year and then saving more.
This isn't a lack of financial education, it's deliberate. People can save but they won't because they don't think they're going to meet the consequences. Or they don't save enough because they don't think they need to. I also know people who say things like "I plan to die the day after I stop working, so I might as well spend everything now."
And that doesn't even consider the large number of people living paycheck to paycheck who literally can't save. It's going to be a shit show in 20-30 years.