Buying periodically doesn't mean you buy on every crash and every day. You keep buying monthly or quarterly when you have money to invest irrespective of the current price. The FOMO comes when you want to buy on every crash.
Also you agreed that you never know how long the crash is gonna go for. What if it stops the moment you sold and start going up? You never know. You can never predict the market. Trying time the market is the worst thing one can do.
I thought you meant buying perodically during the crash. Because this article is about crashes. Most people who make money buy and hold. But the times of crashes can be another opportunity.
You should check out the buyer and seller bars very carefully. If red bars are way more and huge, you can know that the crash is not over. So if you do the research well there, you will make some extra profit. The more you are better analyzing charts, you can predict some stuff.
Search for "Quants". They run the Wall Street and they're mathematicians who see different stories in those charts.