If you have $1 million in cash, would dumping it into a S&P500 Index Fund for 30 – 40 years be the most passive and high return investment?

If you have $1 million in cash right now, would putting all of it into a S&P500 Index Fund and adding $20,000 yearly into it make you a multi-millionaire in 30 – 40 years?

Would that be a better investment method (in terms of effort, time and risk) compared to something like real estate investing?