Supernatural should of ended at season 5. It had a 5 year plan and should of stuck to it. With a bit of editing, the season 5 finale would work as a series finale. People are quick to dismiss The X Files and their later seasons, but at least that show had creativity, new ideas and no rehashing of the same story-lines.
I think The CW have only ever kept it on the air as the longer it remains on the air, the longer it can hold it's record as the longest running american supernatural series.