When the developers of Supernatural began, they only wanted the show to run five seasons. They had a well developed storyline that led to the defeat of Satan and the tragic fate of the Winchesters. Sam gave his life to save the world and Dean got to have a family and a normal life.
Once it was past season five, writers have been shooting from the hip and it shows. The plots have been a little too haphazard and season seven looked like it was put together with a spit and bailing wire. The big bad was mediocre at best and I think the writer's knew it. They focused on other issues and settled everything quickly in the end. Even the big fight was wasn't so big.
So, now were left wondering about season eight. The CW doesn't have a lot of hit shows, but Supernatural is one of them. They'll try and wring every last cent out of it before finally wrapping it up. When Supernatural debuted in 2005, the pilot was watched by more than five million people. It was these ratings that led to its pickup.
The season seven finale was watched by a paltry 1.5 million people. It's lead in, Nikita, was also watched about the same amount. It's going up against Grimm on NBC, which is another show about creatures. Things aren't boding well for the Winchesters. My guess is that season eight will be the last, but we'll see if the CW will let it die out or slowly degrade.