As this isn't a zero-sum game, they could also believe the American economy will do badly but other economies will do even worse.
The parent comments are prognosticating. The way the world looks today is not a good data point when the future includes, what many believe to be, a wildcard President coming in.
And let's not forget the establishment party with a hard on for regressive policy control all three branches of US government. Their entire MO of the last few decades is "no compromises". They've destroyed their reputation as a group (Congressional approval ratings), but don't care because individually they're fine. They've shown no desire to improve their image.
So a wildcard President, and a group of stubborn old assholes is in control. Stick to the guns or spray and pray at the wrong time, and the all time highs could crater.