For decades, ordinary investors with modest sums to invest had limited options. With even low discount-broker commissions making a big dent into regular investments from typical paychecks, individual stocks were largely out of reach, leaving actively managed mutual funds as the primary alternative. Through active funds, money managers got used to the idea of taking 1% off the top on an annual basis, regardless of whether the funds they managed produced gains or lost money for their shareholders. Index mutual funds have been around for 35 years, but they largely coexisted peacefully with active funds throughout most of that time....