ELI5: If 55% of the market is owned by passive investors who DCA into indexes each month, can the S&P500 realistically fall more than 45%?

Sure, some of those passive investors who haven't learned from history (the market always returns to highs eventually) might panic sell their assets, but my instinct is that any panic selling on the passive side would surely be offset by investors and institutions who know to buy into fear.

This may be an absolutely ridiculous question, and I'm prepared to get grilled in the comments (“If you don't know how the market works, you shouldn't be investing,” blah, blah, blah), but I'd love a genuine explainer on where my logic fails me.