When the news broke Tuesday that Megyn Kelly was leaving the Fox News bubble to pursue a broader audience at NBC, my initial thought was: Who cares? Talking heads frequently switch networks, and Americans increasingly get their news not from TV, but from social media sites.But Kelly’s imminent move is huge because it’s Kelly. The very fact that NBC covets her polarizing “brand” is vivid evidence of how profoundly the broadcast media has changed since the kinder, gentler era of Uncle Walter Cronkite.Back then, long before cable news and the Internet, the TV anchorman (always a man) was broadly and implicitly trusted across the political spectrum.