I agree we're not to over indexing in the US, but I think people really have to keep watching the central banks, especially Japan and the BOJ [1]. While the US federal reserve doesn't buy stocks, other central banks do, and a lot of them use indexing/ETFs. The swiss national bank also has a fair amount of US stock holdings.[2]

Given how the central banks act, this affects the currency markets and interest rates.

(1) https://asia.nikkei.com/Markets/Equities/Japan-s-central-ban...

(2) http://www.businessinsider.com/swiss-national-bank-owns-80-b...