There has been lots of coverage of the retirement of Charles Gibson from ABC's World News and the move of Diane Sawyer from Good Morning America to replace him. Nearly all of it centering around "what does this mean for the evening news battle", "second female news anchor", etc.
That totally misses the potentially much bigger story of the economic impact of Sawyer's departure from Good Morning America.
ABC has been a reasonably close second to NBC in recent years in the evening news (as recently as 2007 ABC was #1 in the evening news ratings), but in the morning while GMA hasn't seriously challenged the Today show, they've been a respectable second. In both races, CBS trails badly. (We no longer do comprehensive evening news or morning news ratings posts but you can click the links to see press releases with the ratings numbers.)
What the recent coverage has mostly missed is that those morning shows mean *much* more economically to the broadcast networks than the evening news shows. The morning shows target demo group (adults 25-54) viewer numbers are somewhat lower than the evening news shows, but they're three (or four) hours long vs. 30 minutes for the evening news.
The economic impact of any ratings loss at GMA because of the departure of Sawyer are a far bigger issue for ABC that anything that happens to World News.