aka, reddit's user base.

Reddit user base consists of more than just 20 somethings. And personal volatility tolerance needs to be taken into account when dealing with every recommendation. If you recommend to a 20 y.o. investor to invest in index funds even though his personal tolerance for volatility is low you're basically asking him to pull his money when the market dips.

For example in 2007 if you had invested in the S&P 500 your investment a year later would have been worth half its value. If you're not properly coached and you're not prepared to lose half of your investment you had a very good chance of pulling out (this is evidenced by the average investors return over that time period).

There are questions that could have been asked that could help allocate the portfolio to make it less likely the investor would pull out (Just out of curiosity you should look at where your break even point would have been for 100% S&P fund). Broad recommendations for 100% equity exposure set investors up for failure when the next market correction happens.