I don't think it's a "bubble" per se, since I believe the EFT approach is very sound. That said, there's an interesting relationship between ETFs and speculati

I don't think it's a "bubble" per se, since I believe the EFT approach is very sound. That said, there's an interesting relationship between ETFs and speculation; essentially, the ETF approach relies on the market being highly or somewhat efficient. In turn, the level to which the market is efficient is dependent on people doing their homework and placing informed bets on stocks. Essentially, doing their own evaluations of the "true value" of various stocks. If most investors move towards ETFs, the number of people performing these valuations / bets will decrease, lowering the quality of the valuations. As a result, the efficiency of the market will be lower, and ETFs will be less viable. Or, put another way, having a large part of the market blindly invested in ETFs opens gaps for speculation. The price of various stocks will be further away from their real value, allowing you to have a more reasonable shot at "beating the market" by guessing the real price better.