Both SpaceX and OpenAI's estimated free float are around 4-5% of their shares at IPO. This means that we really are talking about companies in the sub $100M valuation in term of index fund impact (assuming under $2T for each).
But it gets worse because when the lock-up period expires in 180 days after ipo (currently scheduled right before quarterly index rebalancing), it's possible that frees up more than 20% of float and it suddenly has to be weighted at the full 100% of market cap -- triggering additional automatic buying.
It certainly seems like it's set up for our retirement accounts to be the insider's exit liquidity.
In your scenario, that 100% cap would by definition be less than 5x float so it shouldn't trigger any more buying than the lock-up expiration itself did.
Do you mean if free float goes from 5% to 100%, and weighting goes from 25% to 100%, it's more "extra supply" than "extra demand"? That's a good point I hadn't considered. I'm not sure the details of the lock-up period though, it might be staggered. So if it goes from 5% float to 50% float, that's 45% of additional shares available to buy, but an addition 75% of the market cap that indices now need to weight to. But it's true this would only happen once (when free float goes from below 20% to above). Then after that the extra supply would be more than the extra demand. Or do I misunderstand?