At least historically, there is no reason everyone couldn't.

There is no 30 year period in the history of the s&p 500 that returns less than 7% if I remember correctly.

Just imagine what you’d have gotten out of investing in the German, Russian, Chinese or Rhodesian stock exchanges in 1900. I don’t know what happened to people who were heavily invested in the French stock exchange in 1939 but I doubt it was good either. I wouldn’t want to be in the South Korean or Japanese stock exchange if North Korea does anything. You may think war is unlikely but so did a lot of people in 1913. The USA may be the safest available what with the oceans to the East and West and the client states to North and South but it’s not like a civil war would be unprecedented.

Most people don't invest in ultra-cheap index trackers though. They pay high fees for funds that can under-perform the market.

To be fair, index funds are a relatively new thing.