The theory is that when there is sufficient notice that a series will end, you will have the maximum viewing of the target audience to see the end of the story. Basically, you will have a lot of people seeing the last episode regardless. Hence, the writers have no obligation to maintain the quality of the episode. They can make it as poor as they want it to be. That is why I never saw the finale of shows like "Seinfeld" or "Cheers" that had a long run.
Regardless, you will have millions of fans watching Buffy. Hopefully, I'll be wrong.