It's chicken or egg: index funds are successful because people invest in index funds, therefore index funds are...

There will be an inflection point where stocks in indexes are so overvalued that enough active investors (and passive defectors) cause a rebalancing. If they're overvalued enough, and enough of the general market participates in them, index funds could easily crash.

Apps like Robinhood are making active investing much more feasible for your average Main Street person. I certainly am not counting on the continued blind investment in index funds.