Howard Marks provided interesting comment on the index investment. To sum up, he says that too much money has gravitated toward index investment because of its rationality and that would make index investment irrational paradoxically. The process of its cycle is below.
1. Investors want high return and invest actively
2. Active investment could have not outperformed index investment historically; it has contributed to making the reasonable price of equity.
3. As a result of "2." , less investors invest actively and more investors invest in index fund.
4. As a result of "3." , index fund is overvalued and there will be less competition for active investors and there will be a lot more opportunity for the active investors to outperform index fund.
5. goes back to "1."
I took it for granted that index investment performs better than active investment overall unless active investors have special skill set. However, I just recalled the fact that the more things are commonly good the less they are good in the end, as the market is reflexive by its nature.
I would do the best to be an independent, elaborate, versatile thinker to survive in the market.