Inktober Day 1
I think it's worth noting that SPN probably had the first 5 seasons written out well beforehand. (Considering Chuck's books only went until the end of the Apocalypse.) The fanbase's size was unexpected (maybe) but you don't turn down a renewal, so the latter seasons are one-shot seasons that generally stand alone, without a lot of entwining themes. It's easy to refer back to prior seasons for certain details or ideas, but it's not entirely cohesive. Just my presumption, though.