> 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.

This is a common misconception. The S&P 500 weights allocation by float-adjusted market cap, not by total market cap. In the case of SpaceX, they are planning to float ~4% of shares at IPO. Even if SpaceX was added to the index, its index weight would be based on that tiny float, and at a $1.75T valuation it would be treated as roughly a $70 billion company.

SpaceX weight would be ~0.125% of the index, not ~2.5% as you imply.