"I looked at what I think of as the food chain that led to the financial crisis, which was that you had individual consumers buying houses they couldn't afford, sold to them by realtors and property people who were competing to sell more properties at a higher price and so on. [...] I thought, hang on a second, classic economy theory tells you that a competitive marketplace is superior because competition provides a diversity of products which is good for the consumer, and it also, therefore diversifies risk. And yet, in this instance, competition has led every single one of these companies to copy each other, which had concentrated the risk. And I thought, Wow, that's interesting. That's specifically what's not supposed to happen."
More here.
...assuming either a constantly growing economy, or a constant flow of wealth from those without investments to those with them.
I don't think this is actually true. Assume zero GDP growth and zero change in the ratio of income from capital and income from labor. The share price of your stocks does not increase or decrease, but they pay you an annual dividend. The return is still positive even under overall stagnant conditions.