Earlier quoted context omitted.
That "strategy" only works when markets are behaving irrationally, such as they have been for the last 10 years, where basically everything just goes up. If you tried that in 2000 (like I did) or 2008, you'd be quickly destroyed.
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.