Superman in the public domain would be some scary, scary shit.
No, it really isn't.
So long as DC continues to exploit the Superman trademark, even after the early stories begin to fall out of copyright, they're effectively preventing anyone else from using Superman. There's little point in using Superman, if you can't market that it's Superman.
In any case, I expect there will be another attempt in the next year or two to extend copyright protections for another twenty-five years or more at the behest of Disney.