I guess this is somewhat relevant in light of Joss Whedon's mega success at the moment w/ The Avengers.

http://www.warrenellis.com/?p=1786

When the money-losing WB and UPN networks announced that they were pulling the plug to form a single new broadcast network, many television veterans traced the roots of the decision back five years, when a fight over the fate of “Buffy The Vampire Slayer” drove what would prove to be a fatal stake through the WB’s heart.

The show, produced by 20th Century Fox Television, was a runaway hit with teenage girls. But in early 2001, the WB balked when Fox executives demanded $44 million to license a single season. That fall, the show shifted to UPN, and with it went the WB’s identity as the go-to destination for young viewers.

Now, as CBS Corp. and Time Warner Inc. develop the CW, their new jointly owned network, what killed the WB and UPN is a hot topic in the offices of TV executives all over town. In the end, many agree, the WB’s loss of “Buffy” — which breathed new life into the struggling UPN — set in motion a pitched battle for the coveted youth market that would eventually doom both networks…

More to this particular "story":
http://www.thecabinet.com/blogs/blogentry.php?sub_id=horror_blogs&blog_id=thecabinetcom_blog&entry_id=473

The real question, from this article, seems to be "Did Buffy leaving The WB, kill The WB?" I wouldn't say it did, because The WB still produced some great series around that time and thereafter - Dawson's Creek, Felicity, Everwood, Gilmore Girls, Smallville, Angel, Charmed, 7th Heaven - but, yes, losing Buffy in 2001 was a MAJOR blow to The WB. Buffy, Dawson's Creek, and 7th Heaven were the big three that put that network on the map. Dawson's Creek ended two seasons later (and 7th Heaven a few seasons after that). Losing these shows was the end of an era. And the end of The WB as we knew it. They tried to make it work those last few years, but it wasn't the same, and the decline in popularity and ratings reflected that.