But the US has never had $1T+ IPOs before. And also a huge amount of enormous private companies that don't want to go public for various reasons.

Also, the rules have changed before. It's not the first time these rules have changed.

I see both sides of the argument (it's definitely _not_ good for 401k investors if Anthropic/OpenAI/SpaceX make huge leaps in technology that allow for far higher earnings that they aren't able to access, for example).

But my main point is that these investors regardless would "only" have 5% exposure to these. That surely cannot be considered a systemic risk that the OP is inferring.