The fact is that Superman isn't around because he is the oldest superhero. He's still around because he is a great superhero. He's the guy that spawned the entire genre, and that didn't happen because it was a mediocre idea.
We've all read, I'm sure, a Superman book where we didn't really feel like we knew the character. Where the writer, often with the best of intentions, has tried put a personal stamp on the character, whether it be to try and make him more current, or cool, or have a broader appeal, etc.