There have been prolonged stagnations in the market. It took a couple of decades to recover from the Great Depression, and the if you invested in the 60s you might not be doing so hot by the 80s. The market isn’t a sure thing even in the long term, it’s just fairly reliable.

That's true but I'm assuming he continues to work and put in money.

Those events had long periods of stagnate market performance. So they might not lose as much as a full crash, but they might not beat inflation either. Anyways, nothing like that has happened in our lifetime, but it did happen in our parents’ and grand parents’.