So, buying index funds is smart if most other people are trading on information and analysis. You need to be a perturbation on the sea of investors for indexing to make sense. When everyone indexes the strategy becomes nonsensical.
Analogously, high frequency trading provides valuable incremental liquidity when high frequency traders are the governor on the steam engine. When they become the engine itself, they can't provide stability.
The same, of course applied to portfolio insurance in 1987.
Maybe charity has to be marginal too?