The US Market 1981 to present is something of an anomaly that isn't likely to repeat. The same goes for the post WWII era. I'm not saying going long everything is a bad strategy necessarily, but like all investments, it's not guaranteed and has risks. There's nothing wrong with taking value into account when buying investments. It's very different from selling stock out of fear or because of a crash.

counterpoint: http://www.economist.com/blogs/buttonwood/2016/01/investing