The market is always shifting. People who deeply understand the companies in their portfolio will generally outperform the market at large by finding and exploiting mismatches in the market (i.e. active management). The question is how much people are willing to pay for active management. The more that investment moves towards index funds, the larger the gains to be made from active management. Higher demand for active management results in higher fees for actively managed funds, and investment capital goes back to index funds, and the cycle repeats.

If people forget anything about index funds, it's that they're not guaranteed to go up, just like any other investment vehicle. It's a reasonable choice to make to put decades' worth of savings away for retirement, because the stock market trends positive over the long-term, but people are kind of looking at index funds right now as "if I put my money in index funds then I'll be set for retirement because they're 'de-facto guaranteed' to go up over the long-term" and well, that's a dangerous over-simplification.