The popular summary, "time in the market beats timing the market", has a simple arithmetic backing: the best trading days are concentrated into very few sessions, and those sessions usually land inside the panics rather than after them. Anyone who steps out to "wait for things to calm down" frequently misses precisely the days that explain the decade's return.
This is not an argument for ignoring risk. It is an argument about which lever is realistic: nobody has demonstrated a sustained ability to know when to exit and re-enter, and being invested for twenty years has demonstrably worked across almost every historical window.
The practical consequence is boring, which is why it works: choose an allocation you can hold through a 40% fall, automate the contributions, and look at the portfolio far less often than you want to.