Of course, the end of the show doesn't mean the end of fanfic, but it's so much more rewarding to write in a "live" fandom. The fandom is so much more active and vibrant, and the new episodes are constantly providing fresh insights and plot twists and new characters, all of which feed plot bunnies. Plus, I want to experience at least one season from the beginning, following it from the first tiny hints on the blogs of people involved. There is just so much more information out there than there was in 1996, when I got into The X-Files, my last big TV show obsession. It still staggers me a bit that I can read blogs etc. written by writers, producers, actors etc. It wasn't like that ten years ago.
At the moment, I'm quite happy for the show to end after a fifth season, although I may well feel differently next year. I just wanted one more year of "live" fandom to fuel my writing. I certainly don't want to show to go on for too long. I still regret the X-Files, which for a few years I loved with an obsessiveness that I have never matched, before or after, but which I went off so much over the years that I've still not watched the last few seasons.