[walrus66]

Ignoring the obvious controlling interest issues which others have mentioned, which is the largest issue...

That's a funny contradiction of a sort - then the index fund becomes the agent it's supposed to be observing.

For one, Blackrock is not Berkshire Hathaway - and in reality, obviously Blackrock can't wake up tomorrow and decide to be. They're not built for that.

Another scary thing is that the market is being increasingly turned into a derivative, and the underlying asset becomes more volatile (certainly for many different reasons) as it becomes proportionally smaller .