You can't backtest a fund such as VTI. The issue is that these analysis don't take into factors such as volume. I'm sure if someone backtested $100MM investment in Bitcoin starting from 2010, the results would be spectacular, but we all know that buying pressure would've caused Bitcoin to skyrocket and the returns would be less than the backtested results.

I personally tell people to invest in index funds if they don't understand investing, but it doesn't mean it will do better than other options. Index funds are only a recent phenomenon.

The biggest issue with index funds at the moment is "group-think". If everyone in America is investing via index funds then when people need money, those same index funds will fall sharply. There is a valid line of thinking where one should avoid stocks with heavy exposure to index funds because those stocks will have the best returns during an equities downturn.

http://theirrelevantinvestor.com/2017/08/31/today-in-market-...