Captain Marvel is very Supermanesque when looked at from the perspective of a very young superhero genre. People always bring up the differences, like how he isn't an alien or how his secret id is a little boy or how his powers are magic based. But before Superman, there was no character who had his specific set of traits: a super strong hero who wears a costume that includes a cape and chest symbol and has a secret identity. All of those traits describe Captain Marvel, which is why National perceived him as infringing on their copyright. Nowadays, there are countless heroes with those same traits that Captain Marvel no longer seems like a rip-off. Those traits have come to be perceived as generic. I wouldn't be surprised if this is part of the reason for Superman's waning popularity, he's retroactively seen as unoriginal and bland. But hey, that's not the character's fault. It's just the unfortunate consequence of being so influential.