It is evident in the very first comic of Superman that he essentially has no weaknesses. This is exactly what Eco is talking about in "The Myth of Superman" when he says that Superman is mythical. However, you can also see in this first comic that Superman does kind of begin to develop a love life, which is something mortal therefore it is part of the literary genre not the mythical genre. From day one of these comics the writers were toying with the line between mythical and literary heroes. After a long time, the fluctuations become much worse and that is when Eco is saying that the comic is in danger of losing its population. But from day one, the story allowed for a little bit of both mythical and literary characteristics.
I have never actually read a Superman comic so it was kind of cool to see all the different aspects of comics and Superheroes that we have been talking about. The main thing I noticed about Superman was his lack of weaknesses. Nothing could affect him when he was saving the world. In my opinion this is the biggest reason that the popularity of Superman is decreasing. There is only so much that you can write about when the character you have created has only one weakness, and it is too predictable and boring to exploit that weakness every single episode.