Polygon has good coverage of that lecture Gabe Newell gave at the University of Texas School of Public Affairs. According to Newell, the biggest threat to consoles right now is Apple. Here's a quote:
The biggest danger facing the success of Steam Box or any other PC ecosystem hoping to find space in the living room is Apple, according to a lecture given by Valve co-founder Gabe Newell to a class at the University of Texas' LBJ School of Public Affairs.