You want my opinion, you say? Here it is. You're better off investing in index funds than looking for some other tactic, and despite the headlines, these articles contain evidence FOR investing via index fund.
Details:
The first article states that the biggest person offering concern, indexing saint John (Jack) Bogle, didn't think it would be a problem any time soon. He and the article also gave perfectly good reasons why. In the fine print, they noted that small companies might have a slight edge if all this is true. Your actionable conclusion should be to possibly buy a little more small-company index (Russell 2000 index) fund, and a similarly little bit less big company index (S&P 500).
The second article gives evidence suggesting that the index-funds-will-collapse idea is wildly unlikely, because the gains that index funds rely on are largely based on aspects of human nature that measurably do not appear to be changing. Your actionable insight is to invest with confidence, ignoring the fear-inducing headlines that get attached to even the most mundane of articles in our clickbait media sea.
The third article is designed to suggest paying an investment advisor to invest for you. The ideas presented are weak in supporting the article's thesis. They assume that investors will sell at the bottom a bear market, but leave their money in an advisor's hands. I think that most people who would sell after big losses would do so whether or not an advisor is involved. They also assume that the advisor will stay calm in crisis. That's far from guaranteed, but the advisor's fees ARE guaranteed. My guess is that you're better off picking index funds instead of managed funds, or instead of buying various funds through an advisor. In any case, the article has little to do with any hypothetical collapse of index funds; it really just discusses investor behavior during market plunges. Market plunges are inevitable, outsized collapse of index funds is not. Your actionable plan should be hold on to stock during downtowns... presumably stock held in index funds.
The fourth article has a section title that appears to address the index question directly. But the body of the article doesn't. It's not about indexing at all. It's just a long standard article saying that future returns are likely to be moderate rather than high if prices return to their long term historical averages. This is probably true, but irrelevant to whether index funds are a good vehicle for any stock investing that you choose to do. The author herself stated that her main investments are a set of index funds and/or ETFs. For purposes of your question, ETFs are a variant of index funds. Based on the article, your actionable moves should be to invest in index funds, include a large-ish % of stock funds rather than bond ones, and don't give up just because you experience a dip for a few years, or a decade where your profits are small (say 20%) instead of large (doubled your money).