There's lots out there about how cheap and automatically amazing index funds are: they buy everything so they get reasonable exposure to everything... individual savers don't have to care, be experts or even know anything about what they're doing. BUY ALL THE TIME and you'll be fine, basically.
BUT... there's also judgment involved in which stocks to exclude from the index; and the timing at which you "switch" indices or choosing which index to follow:
The ethos of passive investing is taking the good with the bad. Intervening when it suits you seems to some almost like playing with nature, but Schneider points out that there’s nothing sacrosanct about popular benchmarks.
Sometimes flying blind can be a good thing. For example, there are very few people who paid a split-adjusted one dollar for a share of Amazon and had the discipline to hang on by the time it had reached $250 a quarter-century later. Index funds stuck with Amazon through thick and thin.
"Even smart (or lucky) professional active fund managers don’t do that. They can’t—professional risk and concentration rules force some selling once companies get huge."
As long as a stock continues to tick the boxes, indexes are too dumb to sell it. That, aside from low cost, is one of their advantages. The market’s long-run return is driven by a small number of massive winners that more than makes up for lots of duds.