It's not just about returns, it's also about risk. The role of a passive index fund is to be a passive index fund. If the s&p starts chasing returns, that will reduce its utility to the market. You get higher returns by being compensated for risks that passive investors/retirement funds don't want to take. And active investors use the S&P and similar indexes for the specific risks and asset class exposures they provide. You might think the economy is going to do poorly which would be good news for some company that's anti-correlated with the economy but you need to hedge that bet for if the economy does well, so in addition to buying shares of whatever that company is you buy into some market reflectiong mix of stocks bonds etc. The role of that hedge is to have a counterbalancing asset that moves opposite your primary bet to reduce volatility, and the role of the s&p 500 is to broadly reflect the american large cap publicly traded stock component of the market. If the S&P 500 begins behaving unpredictability to chase returns as an index then buying funds that track that index is no longer doing what you need it to do. S&P index loses utility, active investors just use some other index, but passive investors with 401ks locked in to tracking the s&p are suddenly forced to buy whatever bet the index creator is making en masse driving the stock price up. That's not a good outcome for anyone except the company muscling their way in and anyone that was somehow rewarded by that addition