Don’t forget putting a lot of money in control (not own, but control) of a few investment companies. Vanguard and Blackrock could shift the entire market by choosing how their indicies get allocated. Most of that money came to them by way of 401k ETFs.
Now, if you have a 401k, putting it in an ETF is invariably your personal best option (for the proportion invested in stocks, anyway). But there are implications here when everyone does it.