This is a bit tangential, and I'm not a legal expert, but as I understand it, there's a difference between copyright and a trademark.
WB would not lose the copyright to the stories printed in, say, Action Comics, if they stopped making Superman comics. But over time, they can lose the registered trademark of his name, which would open the name up to being used by other parties. In fact, this very reason was given by writer Paul Kupperberg in one of the early issues of The Daring New Adventures of Supergirl (in 1982) for why they couldn't change Kara Zor-El's hero name from SuperGIRL to something more befitting a grown 20-something woman, like SuperWOMAN (quite aside from the fact that there already was a character by that name, and it wasn't Kara). He took a bunch of flak because he de-aged Kara from mid-20s to 19 years old, because it made more sense to call a younger female "girl." Fans said he should just change the name, and he said DC couldn't because they'd lose the trademark if she stopped appearing in print for too long under that name. The rule with trademarks, he claimed, is "use it or lose it."
Wikipedia seems to agree, stating this about Trademark registration:
"Trademarks rights must be maintained through actual lawful use of the trademark. These rights will cease if a mark is not actively used for a period of time, normally 5 years in most jurisdictions. In the case of a trademark registration, failure to actively use the mark in the lawful course of trade, or to enforce the registration in the event of infringement, may also expose the registration itself to become liable for an application for the removal from the register after a certain period of time on the grounds of "non-use"."
The article goes on to say that
"In the U.S., failure to use a trademark for this period of time, aside from the corresponding impact on product quality, will result in abandonment of the mark, whereby any party may use the mark."
(I'd post the link but I can't due to posting restrictions, so go to their entry on Trademarks to read the full text.)
This entry seems to agree with Kupperberg's claims in the 1980s, that DC had to keep putting its characters into print under their TMed name on a regular basis or they risked losing the registration of the mark, whereupon Marvel, IDW, or anyone else could register it. Note, Marvel couldn't make up a character in a blue and red costume with an S on her chest named Kara Zor-El, from Argo City. But they could have made up a new X-mutant with psionic powers called "Supergirl" if DC stopped printing the character. And register it. Thereby prohibiting DC from using the name later.
I suspect this rule, more than any other factor, is a major reason why we keep seeing characters with trademarked names get resurrected and "never stay dead." If they kill a character and leave it dead for decades, someone else could grab the mark. So they kill Barry Allen, but Wally West is still called "The Flash" and so on. If they had kept Wally as "Kid Flash" instead, they'd have lost the rights to "The Flash" eventually due to lack of use.
So, copyright and trademark are a bit different.