In a number of articles I've read about Jimmy Fallon taking over Jay Leno's slot as the host of The Tonight Show, much is made of Fallon's friendliness to the idea of breaking the show down into small clips that play the next morning on the Web. The discussion then becomes about the pros of that approach (makes content available the way the younger generation wants to see it) versus the cons (arguably disrupts the core franchise). Either way, the focus is on the idea that Web video and time-shifting potentially undermine the value of The Tonight Show franchise by making it easier to watch bits of The Tonight Show outside its core time slot.
The real threat seems to me to be different. If I think about why I don't watch David Letterman nearly as frequently as I did 10 or 15 years ago, the answer is that back then he had the best show that was on during that time slot.
And time slot effects were core to Jay Leno's success. Leno has never been the tastemakers' favorite comedian. But The Tonight Show airs at 10:30 p.m. in Central time vs. 11:30 p.m. in Eastern and Pacific time. Since more people watch TV at 10:30 than watch at 11:30, appealing to the Central time taste profile is more important than appealing to coastal elites.
But nowadays the whole idea of watching a television show because it happens to be on at the moment seems impossibly quaint. If it's midnight and you're awake and want to watch television, you do the same thing you would do if it was 8 p.m. You watch what you want to watch! You fire up the DVR or Hulu or Netflix or HBO Go or whatever, and put the show on. Who cares what time it is?