In 2019, Bayer acquired Monsanto, turning the S&P 500 into the 499, leaving a gap. Was company number 501 bumped up and into the S&P 500 to replace Monsanto? I don't know, but I doubt it. Twitter was added to the index; I doubt it was number 501.
Less than three years later, Twitter is leaving the S&P 500, and some other company will be added. A bunch of index funds will be buying that new addition, not because it is a good entry point, but because Elon Musk is taking a different company private.
We invest in index funds to try to match the market instead of trying to beat the market. But what is "the market"? If you own a 500 index fund, the market is those 500 stocks, in a weighting determined by Dow corporation. You no longer do stock selection, and your fund manager no longer does stock selection (reducing fees) but instead you are aping Dow's stock selection. The hope is we all aim for average. Even if you pick a total market return fund, there still is that weighting selection someone else is doing. We get both oil and tech, but we are not deciding how much oil and how much tech.
I have heard many finance professionals make statements such as "we can't beat the market". But the market is an average, by definition. It's the ECON 101 guns vs butter trade-off. In the ideal world, half would beat the average, half not. In a world of index investing, everyone is average. The lure of index investing is that the financial advisor making the stock selection charges a fee and yet is no smarter than you. So you can increase your return by his fee that you saved. But...
You could do the stock selection yourself. If you decided to put a portion of your funds in a 500 fund, and another portion in an international fund, you just did stock selection. Choosing those portions was selection. At the other extreme, you could own individual stocks, no funds, and have zero fees. Vanguard and Fidelity have some zero or near zero fee funds.
Where is this leading? Dow will announce the addition of a stock to the S&P 500 following the removal of Twitter. Dow will also determine what weight in the 500 that stock will take. Index funds around the world will be buying up that stock—not because it is a good time to buy it—but because Dow selected it. All of that concentrated buying is artificial demand, on an artificial timeline, and, ECON 101 again, drives up the price. Note that the value of the company has not increased, only its price. After this artificial demand is satisfied, we would expect the price to revert to its value, and each index fund to have lost value on its purchase. "The market" is artificially reduced.
I'm not a fan of index investing, to the extent buys and sells are not based upon economics.