If you put your money in the stock market over the past decade, it's vastly over-simplified to say that your return would be zero. It's highly dependent upon what you invested in. I know many people (myself included) who have fantastic returns over a rolling ten year period.
Of course "it depends upon what you invested in" but your parent comment is referring to the market average or equivalently the return of an index fund. That's the only way it makes sense to compare the returns of different time periods. After all, in any period there are stocks that offer large returns but that of course doesn't make the stock market a sure bet since we only know the good stocks in hindsight.
