I agree with you. He is the archetype of a superhero. He is THE superhero. He is also noble, good and cares about other people.
As for the reasons he is not so popular nowadays, well I think that he had the last decent movie like three decades ago is a good one. Another one is that this are dark and cynical times. People are not so innocent as in the 70s or 80s. For that reason, people find antiheroes or flawed heroes much more interesting. Definitely Superman is a role model for more positive and optimistic times, but that's the reason we need him so much nowadays. To have a reason to believe that being good is possible and that there's someone that kids can look for when they need someone to admire and to try to be like him when they grow up (although sadly without that kind of superpowers)
In my opinion, the fact that Superman is so "perfect" is very interesting for a story. We can see how always being right can be impossible, how he may have dilemmas when having to take a difficult decision, for example having to choose one of two people to save (similar to Batman in TDK) or struggling not to kill a villain, etc.
While Batman is interesting because he is tormented and fights crime without superpowers, Superman is interesting for other reasons. And he is a tragic hero! He lost his family and all of his planet! Isn't that tragic enough for you?
captcha: do not touch