Traders work on the floor of the New York Stock Exchange shortly after the market's opening in New York

As Wall Street hit another new record Thursday, it is worth considering what could cause a serious setback in stock market prices around the world. Since I started writing this column in 2012, I have repeatedly argued that the rebound in stock market prices from their nadir in the 2008-09 global financial crisis was turning into a structural bull market that could continue into the next decade.