If you're willing to educate yourself on how to pick good mutual funds (Morningstar is a great resource), you may as well use index funds. However, you want to diversify outside the S&P 500 to get exposure to, e.g., international stocks, small-cap stocks, value stocks (the S&P is tilted to blend/growth - see https://www.morningstar.com/funds/xnas/vfiax/portfolio), etc.