The very nature of saving in a 401k is that you never at one time put in a high percentage of your balance into it. The max individual contribution is $18,000 a year.
Let's say you have been doing around $10,000 the first 10 years of your career and the economy tanks. You now have 30-40 years until retirement, and you start doing $15,000 or so a year. That money that you put in before this huge crash may not grow that much before retirement, but the money you put in after the crash will be bought at low rates and will grow for a long time. You will also have put in a lot more money post crash anyway.
And you can also do other savings vehicles outside of your 401k to save even more money. I just wanted to illustrate that it's not like people put all of their money into their retirement account on one day and if a crash happens the next they are screwed.
True, I haven't been 401k eligible h(or even Roth), so I have a lot of savings in post tax cash. I just got my Chinese pension contributions refunded, at least, but I'm way behind even on my social security contributions.
Let's say you have been doing around $10,000 the first 10 years of your career and the economy tanks. You now have 30-40 years until retirement, and you start doing $15,000 or so a year. That money that you put in before this huge crash may not grow that much before retirement, but the money you put in after the crash will be bought at low rates and will grow for a long time. You will also have put in a lot more money post crash anyway.