Is there a “self fulfilling prophecy” aspect to the SP500 performance?

New investors hear from others and read in their research that they can simply buy the SP500 to outperform most other strategies, so they buy an ETF of it and DCA it over the course of their life.

Buying an ETF causes the price of it to go up, contributing to its performance.

So then, can we attribute some of the SP500 famed performance as simply due to it’s popularity?