Index investing has really only existed in common practice since the 1970s. You can simulate back further and do imaginary index investing, but we really only have 50 years of actual history with it. It's a very young experiment.

One thing that a lot of people are worried about is if the surge of people and money getting blindly pumped into broad basket index funds as if it was a savings account (because those have negative real yield) will itself distort the market in weird and unpredictable ways. It's entirely possible that it breaks the entire system and creates the mother of all crashes.

This, pretty much. Whenever a new trend comes up, be it Bitcoin or index investing (both of which are poles apart in terms of risk), passionate people will defend them passionately. Good on you all. I posted it hoping to see HNers' original or novel thoughts about this (along with the expected defending of index investing).

It's pretty tough to figure out if there's over-exposure in index investments, and tougher still to untangle the expected fallout of that problem, if it exists. Some people have talked about it, and have been shouted down of course.

I've done a decent amount of reading on the topic and think I'm barely knowledgeable about the surface of it. I guess that's how it always is, though.

People burying their heads in the sand and thinking that index investing has no hidden black swans are the ones to be most scared of, though.