I think TV series must not exceed 10 seasons. Supernatural is a good example of this. When Supernatural started in 2015, it was really entertaining with an interesting story and scenes. The story was mainly around death of a father and two brothers seeking for revenge. Killer was the yellow-eyed demon however it was not the only rival, they had to fight with many other monsters as well. It was not only fiction but also included lots of humor as well. While Dean was always enjoying the situations however Sam was very much focused on their purpose.
After three entertaining Supernatural seasons, they have reached their ultimate goal in Supernatural season 4 and they got their revenge from the yellow-eyed demon. Mission was complete but the series was not complete. Can you imagine main story ended at season 4 however it is season 13 nowadays?
I have to admit that any show continuing 13 seasons should be good. Somehow producers of Supernatural have managed to continue to the show despite declining quality and followers. Especially Supernatural season 12 was the most disappointing one so far. It is normal that quality started to decline as all the possible stories are told already, there is nothing left.
Let’s see how they will end this great TV series..