Study "the lost decade" Nd it's reference to the S&P 500's net zero performance from 2000-2010 due to two big corrections. Had one been in an S&P index (which is arguably the most popular index) during that time and counted on needing growth for retirement they would have missed out on how well international stocks and bonds did during that time.

However if you're 25 and have many years to work with and are comfortable with it's inevitable swings then you can make a great case that yes it's a good place to be.

As one closes in on retirement, diversification is key. You can diversify through indexes but that requires buying several different indexes and not one like most index investors do.