Correction: index funds don't have a choice. They must follow the index, and so must buy the stock.
side effect: they'll have to sell other stocks, pushing their prices and weighting in market cap weighted indexes down.
> Passive investors are unable to rapidly respond to these types of changes because liquidating portfolios will incur capital gains taxes. (?)
For some active investors, yes. For passive investors (say you through your employer's pension fund), the tax isn't the problem. It's that the market has such a short time to adjust the price of these companies before indexes are forced to include them--and so might buy them at wildly inflated prices. Then, not too long after, the early investors can sell at still-high prices as soon as their lockup periods end. It's a massive transfer of wealth from pension funds and index investors to the early investors in those companies.
Maybe, most indexes do not have to follow the index. they just need to match the returns. An index fund manager has choice of what stocks to buy. However an index fund doesn't have enough managers to make many choices and so they normally buy just what is in the index. However all index fund managers know they are large enough that if they change their holdings "instantly" when the index it self changes the market will collapse and so the fund will under perform. Thus index fund managers are always trying to figure out what the index will do so they can start buying/selling stocks in smaller amounts before the change happens.
How each fund handles this is up to the managers. (and "total market" funds have less ability and need to do this)
Just look up the performance of Mutual Funds vs S&P500.
This is a great technical point, and in a scenario where a constituent has a lot of obvious correlations it might be relevant, but when you've got something that is effectively a meme stock with erratic leadership and a huge range in possible outcomes from bankrupt to most valuable company ever in the universe, I don't see how you promise to match the performance of an index where it's a significant component except by buying it.
On the other hand, do you want to be the one who says, "As a rule we follow the index, but this time we decided to break our own rule, and as a result we lost X% of returns"?
Better wrong with everybody else than wrong on my own.
Right, if they've advertised as an S&P 500 index fund, they have to robotically follow the S&P 500, stupid inclusions and all. Changing that strategy would require ... a lengthy process involving input from shareholders.
However, someone can still start e.g. a "classic S&P 500" fund that follows the old rules for inclusion, and I suspect we'll see that in response to these recent decision.