I don't think citing The X-Files is actually a good argument for keeping the show going. TXF was gasping for air before it crawled to the end and should have been ended years before. Carter had several opportunities to end the show gracefully. From the Black Oil arc that was just being made up as he went, to Duchovny vanishing intermittently, then Anderson following suit ... it was just floundering and barely able to justify its existence as it lurched from one year to the next not knowing who would be in the cast or if the black oil would ever make any sense. If the show had brought in Patrick and Gish much earlier and made a clean break with Duchovny and Anderson it would have been a lot more cohesive and a lot more entertaining to watch.
At least Supernatural has the original duo, but there is no way I would want this show to go down the same road of TXF just to keep it going.