I think the big moment, if it comes, is when you can envision the difference between how people value the asset in question and how that asset is actually value

I think the big moment, if it comes, is when you can envision the difference between how people value the asset in question and how that asset is actually valued. In the case of the housing market, most people in 2003 pictured suburbification and the generation of baby boomer retirement communities as inevitable economic engines. But in the same era, Arrested Development the TV show was skewering over-production of prefab houses and they were closer to the truth. If you could make an analogous case today that the majority of the companies that make up the S&P 500 are over-valued then you could presage the popping of the ETF bubble. In this case, however, the population size of the S&P 500 is much easier to monitor than the population of houses in the US Housing Market, which in 2003 were being built far from Wall Street in pockets of Arizona, Nevada, and Florida. In addition, companies can be subbed in and out of the index with ease in a way that houses cannot pop in and out of the market. That is why I think the housing bubble pop in 06/07 is not analogous to the current era with ETFs