Hi Travis, thanks for reading. Your caution is correct — as I noted in the section titled Caveats, there’s no guarantee the market grows every decade (see: dot com bubble and housing crisis). However, if you continue to contribute every month, even in periods of recession, you will catch the market at its low. The money you invest during these lows will generate substantially better returns when the market recovers. Remember, the goal is to hold for multiple decades.
The premise of this article takes the S&P500’s average over the last several decades, which takes the recent recessions into account. However, no one knows how the market will behave in the future, which is why it’s wise to invest steadily so you make sure to catch the valleys.