As we saw in the 2008 recession when more and investors pile on even the safest investments (mortgages backed securities) the quality of those investments tends to be diluted (via CDO's, errant ratings companies, exploitative real estate practices)
This is what's making me nervous about index funds right now, the demand seems extremely high, but what about the supply how would you determine such a thing?
What is investors are smart but there simply aren't enough good investments out there? Are they going to give up and become doctors, lawyers or engineers? Or just spend more time with their kids?