In 1938, when Jerry Siegel and Joe Shuster sold their Superman creation to National Publications (later known as "DC"), the existing copyright laws of the USA said that no copyright could last for more than 56 years. You got a 28-year term of copyright protection to start with, and then, if you filled out some legal paperwork at the proper time as that 28 years was running out, you got to renew for a second 28-year term, and that was it. So as far as Siegel and Shuster knew at the time, there was no reason to think anyone would have exclusive control of "the right to publish new Superman stories" after the year 1994.
During the 1970s, Congress changed the rules for copyright lifespans. Where "old copyrights" were concerned, the 56 years of protection could now be extended for another 19 years, bringing it to a grand total of 75. Which meant, at the time, that the original copyright on the stuff published in "Action Comics #1" was then expected to drop dead in the year 2013.
Many years later, Congress changed the rules again, and as it now stands, the copyright on the earliest Superman stories will only run out after 95 years. Which means that material first published in 1938 is expected to enter the public domain in 2033 -- unless Congress changes the rules again before that time!
Over the years, I've sometimes seen fans suggest that it would be a good thing if Congress, in the 1970s, had only changed the rules for "anything new that becomes copyrighted from now on," but had left the rules about "old copyrights" strictly alone! (Also, some people think Congress should not have changed the rules at all in the 1970s; not even for stuff that hadn't been created yet; but we'll ignore that for now.)
So I finally decided to take a poll of my fellow fans. How many of you think it would be better if Congress had left the rules alone, with a strict 56-year limit for exclusive copyrights on all stuff produced before the late 1970s? This would mean that by now, Superman (and all other Golden Age comic book heroes) would be in the public domain as far as U.S. copyright law was concerned, and we'd even be starting to see that happen to early Silver Age characters. (Anything originally copyrighted in 1959, for instance, would be hitting its "expiration date" and passing into the public domain during this year; 2015!)