The impact of national network broadcast news, in the digital age, has dwindled substantially in importance and reach – so much so that tonight’s latest changing of the guard, from ABC’s Diane Sawyer to former ABC weekend anchor David Muir, also comes with a reduction of power in the primary prime-time anchor seat. Before tonight, on broadcast TV, the evening news anchor was the one who led virtually all breaking news and election coverage – but under the Muir regime, those duties will go to Good Morning America and This Week host George Stephanopoulos, who’s been given the title of “chief anchor.” The move may require more teamwork and deference than ever before at ABC, but it’s also a shift aimed at a slightly younger demographic: Muir is 40, Stephanopoulos 53, and the departing Sawyer is 68 – three years older than Walter Cronkite was when CBS took away his chair and gave it to Dan Rather.