The $70-billion-a-year television business (in the U.S.) has been under attack from all sides—Amazon, Netflix, YouTube, and other services are all stealing attention (and revenue). But amid the shift to on-demand entertainment, traditional TV has doubled down on what only it can offer: live events, particularly news, sports, and weather. Easy-to-use, mobile live-streaming services could upend what has been the last sacrosanct aspect of the TV industry.