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Does longterm investment in index funds still make sense in a reality of massive algotrading?
What the automation mostly does is make short-term trading that much more difficult. Day trading is a zero-sum game, so if they win more, everyone else wins less.  Long term trading (years to decades) is a positive-sum game; the market as a whole tends to move upward for fairly obvious reasons (at its basis it's still investing, which in turn is based on lending, and as long as folks make fairly rational decisions about how much return they demand for their investment and the companies are mostly producing profits there will be a share of the profit coming back to the investors as dividends or increased share value or both. Day-to-day churn in individual stocks gets averaged out by diversification and time, and by the assumption that if you've waited that long you can wait a bit longer if necessary for jitters to settle out. Time periods between those will partake of some mix of the two.