Network Anchors Hold Fast to Their Dwindling 15 Minutes
Roles of television anchors Peter Jennings, ABC News, Tom Brokaw, NBC News, and Dan Rather, CBS News, seem oddly diminished in time when there is little breaking news at political conventions; all appear resigned that they are no longer giants as they fight for prominence in media world crowded with newcomers, political world where conventions are scripted and corporate culture that is unwilling to give them more than one hour each night for coverage; graph shows television ratings of conventions from 1960-2000; photos (M)