Be careful of projecting future stock (and bond) returns based on historical averages. According to Bloomberg Businessweek reporting on a study by McKinsey & Co, market returns will be much lower over the next 20 years compared with the past 30. The report predicts average annual returns of 4% to 6.5% over the next 20 years, compared with an annual average of 7.9% in the past 30 years. Check out the graphic "Lowering your sights" at: http://www.bloomberg.com/news/articles/2016-04-27/be-afraid-be-very-afraid-if-you-re-investing-for-the-long-run#media-1
For a more detailed discussion see: http://www.bloomberg.com/news/articles/2016-04-27/be-afraid-be-very-afraid-if-you-re-investing-for-the-long-run
The report's conclusions confirm academic research that projects lower domestic and international stock and bond returns in the coming decades. So, bottom line, we need to save and invest more to ensure a secure retirement.