> sensible thing to do is to create a new index with the new rules

It depends. Indices aren’t funds. They aren’t meant to balance investor interests. They’re meant to communicate some metric about the market.

The S&P tells you how big companies are doing in an index optimized to balance representation against trading cost. So in 2005, float was taken into account for weighting (versus just market cap). This made sense. Also, since the start, the S&P 500 has been a committee-based index. Not rule based. This has made it successful; if you want stable and unchanging, you never went for the S&P 500.

The S&P 500 may not be a fund itself, but Standard & Poor's is a business whose ability to sell services is correlated with the continued relevance of the S&P 500. It absolutely does balance interests - namely, its own - beyond simply being an academic vehicle for communication of a stable thesis.

It seems entirely reasonable to say: "if we make a certain decision, we correlate both our reputation and a nontrivial portion of the U.S. economy with the whims of one of the most volatile personalities in industry, and we should likely pay attention to this trial balloon that shows such anticipatory fear of the decision that we might lose our reputation as an index altogether."

> They’re meant to communicate some metric about the market.

Is that why people spend time, money and effort creating and maintaining them? They're just broadcasters? That seems dubious.

an etf that tracks the S&P 500 is what then?

This is a big win for many S&P 500 etf holders

The market cap of the S&P 500 according to Google is ~$65T. Anthropic, OpenAI and SpaceX could well amount to $4T+ in market cap. That's ~6% of the entire index. It's like adding another NVidia. That's a big deal.

The rules around index inclusion exist for a reason. Too much control in one person's hands (which SpaceX has), too small a float (so you don't get price discovery), lack of a history of financial performance and minimal trading days just don't give investors confidence and, like it or not, investment decisions are made based on the index. If you want to argue against passive investment, well, good luck with that.

I think a lot of people have this weird idea that what we need is some theoretically unfettered market for "true" price discovery when it's actually regulations like this that create markets. It's like a libertarian brain worm.

I don't think anybody wants these mega-companies out of the index, specifically. They just don't see why rules that exist for a reason should be suspended when the net effect of that is that investors have less information and there is a lot of forced purchasing. If you have confidence in your IPO, let the market decide what it's worth without trying to fix the price because what they seem to want is for insider lock-ups to end about the time we'd otherwise be getting normal price discovery. Kinda weird.

Investor confidence needfs to be managed by creating a stable, regulated market.