Earlier quoted context omitted.
The stock market is less than 200 years old. Let's not make any bold predictions about unbreakable trends when we're talking about time periods up to 25% of the age of the market itself. After all, at one time the conventional wisdom was "No football team who plays their home games in a domed stadium will ever win the superbowl." And there were decades of evidence to support that statistical fact!
There are real forces that propel the stock market upwards: productivity and an ever increasing population (so more overall revenue and therefore profits over time). This is why its a pretty safe bet that over 15 year+ time periods that the stock market will go up.
The U.S. population growth rate has also been in decline for the past 50 years, dropping from 1.7 percent to 0.7 percent, heading towards negative growth if this trend continues.
So much for "an ever increasing population" to bolster a market that always goes up.