Here's a question for someone more savvy than me. What happens if the vast majority of stock investments end up in vanguard funds? In an economic crises, will everyone try to sell the same set of funds and will crash the funds themselves?
This doesn't answer your question directly, but Matt Levine has written some great articles, in particular, about index funds and their effect on the market. Here's one: [1] "Second: One of my little stock-market obsessions is that index funds free-ride on the work done by active investors. Someone needs to make decisions that allocate capital to businesses. A world in which everyone indexes, and in which no one thin…
A tangentially related point is that membership in e.g. the S&P500 imparts a big financial advantage to companies for a somewhat arbitrary reason, due to passive investing.