The business cycle is a thing. Passive investing means expecting that, and not selling at the bottom. (For short-term liquidity needs, you shouldn't be in the stock market at all).
There are also Black Swan-like events of stock exchanges and entire markets disappearing completely, like it happened in Russia after 1917 and in China after WW2.
If something like the Russian revolution comes along then all bets are off, it pretty much doesn't matter what you did with your money as long as you didn't flaunt it.
I think I could argue both sides pretty convincingly. It is easy to see in hindsight, not so easy in the thick of it.