Megan McCardle, one of the Professor's three guest hosts this week writes what a milestone Jennings' passing away represents:

Many people spent more time with Peter Jennings than with their adult children. I don't watch television news except during big disasters; I find it too shallow and graphic to be useful. But for many people, Peter Jennings was their point of contact with the wider world.

And his death represents the end of an era. No one will ever occupy the place in the world that Brokaw, Jennings, and Rather did. Americans are no longer limited to three channels, nor forced to take their news in discreet bites between 5-7. The world is probably better for it, but something--if only a connection to our past--has been lost.

Howard Kurtz wrote back in 2002 that this moment was coming, when all three of the anchors would be gone, the men who dominated the TV news during the Big Three networks' transistion from a monopoly to but one of dozens of choices viewers now have thanks to cable, satellite, and talk radio. And now, thanks to the Blogosphere, the news and opinion choices of the public are infinitely more crystalized. (As you've already discovered, if you're reading this!):

For two decades now, through war and scandal, through serious times and silly times, Dan Rather, Peter Jennings and Tom Brokaw have ruled the television world from their network thrones.

Their companies were bought and sold, their audience distracted by new choices and new technology. But no one dared mess with America's anchors.

Now, with little warning, that may be changing. In an age when even Ted Koppel is deemed expendable, when ABC is willing to replace "Nightline" with funnyman David Letterman, the once-sacrosanct evening news suddenly seems vulnerable to bottom-line executives at Disney, General Electric and Viacom.

As the 70-year-old Rather, 64-year-old Jennings and 62-year-old Brokaw head into their sunset years, the programs will no longer be shielded by their prestige.

"When Brokaw, Jennings and Rather retire, it is a perfect time for these corporations to decide their newscasts are no longer worth it," said Ken Bode, a former NBC correspondent who teaches at Northwestern University's Medill School of Journalism. "Unless something dramatic happens, inevitably, the network newscasts are gone."

If that happens, it would be the biggest change in news consumption in the half-century history of television, an erosion caused in part by the striking failure of these programs to attract viewers younger than 50. Thus, they have the same problem plaguing "Nightline" -- an aging audience that is gradually dying off.

For some thoughts on the rapidly graying demographics of TV, click here. And for some thoughts on how that will impact advertising, the mother's milk of mass media, click here.