Does the popularity of indices (e.g. SP500) affect their long-term performance?

When discussing risk-adjusted returns there is a lot of literature, like factor investing, focusing mainly on back testing.

However, there seems to be a new trend, which will likely only increase, where equity investing is becoming more accessible, and many people and fund managers are simply investing in some well known indices. Perhaps the main example is the SP500, which gets a lot of investment regardless of the performance of the companies.

If we assume that this trend will continue, meaning that more people will invest in the stock market, and that a significant part will blindly invest on a given index (let's say SP500): does it mean that SP500 is more likely to outperform other stock groups in the future?

Clarification: I am not referring to the impact in individual stocks of being included or removed from a given index. I am referring to the performance of the index as a whole.

Previously asked in r/askEconomics: [https://www.reddit.com/r/AskEconomics/comments/1isn99j/does\_the\_popularity\_of\_indices\_eg\_sp500\_affect/](https://www.reddit.com/r/AskEconomics/comments/1isn99j/does_the_popularity_of_indices_eg_sp500_affect/)