Thank you sir.Actually you as an investor can "create" a share of SPY by depositing a basket of stocks with them. Effectively institutions arb ETFs when demand for the ETF exceeds supply and create new shares in that manner (and can also do the opposite). So unlike a mutual fund, the ETF sponsor isn't out buying and selling underlying (for SPY at least). Detailed explanation in the prospectus.
A hypothetical question:
As the percentage of money investing in indices approaches 100%. What happen to the market? Would it be more or less volatile?