No, we should expect the opposite: an increase in leftwing terrorism.
The DHS report seems written from what we might call the romantic model of terrorism in which terrorists feel marginalized and frustrated and resort to violence out of a sense of desperation. In reality, terrorists are megalomaniacal sociopaths. They engage in terrorism to fulfill their desire for fame, adulation, respect and power. They fantasize about their violent acts leading to a world in which they are hailed as heroes for their deeds.
Timothy McVeigh expected to set in motion a chain of events that would lead to a militia-style revolution which would lead to a world in which he would be seen as a revolutionary hero. Bill Ayers fantasized about leading a communist revolution that would end up with him as a Stalin or Mao.
In short, terrorists don’t engage in terrorism because they believe in causes. They believe in causes because the causes provide a rationale for carrying out their fantasies.
This desire for power and fame means that terrorists chose extreme versions of the dominant causes popular at the time. This means that rightwing terrorism is prominent during eras when rightwing ideas are ascendant and leftwing terrorism is prominent when leftwing ideas are prominent. Militia-group terrorists were active in the ’90s when rightwing ideas were ascendant and leftwing terrorists were active in the late ’60s-’70s when leftwing ideas were ascendant.
This means, if the recent election represents a political shift of America to the Left, that we will see an uptick in leftist-“themed” terrorism. I suspect the real growth industry will be an acceleration of the already ongoing ecoterrorism, although terrorists watching the villification of those in the financial sector might see targeting those people as the road to fame and power.
Of course, given the track record of leftist-themed terrorists from the ’60s, who all landed on their feet to become respected members of the Left, we might see more terrorists than history would lead us to predict. A subculture in which being a terrorist gets you tenure and book deals doesn’t exactly communicate the message that the subculture finds political violence unacceptable. This tolerance for terrorists will make terrorism seem more acceptable to future wannabes.