I thought it was just the steamboat cartoon itself that was public domain - didn’t they use Mickey mouse from it as their new animated studios logo (which extended the rights to it?)
It is just Steamboat Willie in the pubkic domain now, including Mickey Mouse as a character, although newer Disney interpretstions of the characters (like gloves) are still under protection. And while Disney can use the original Steamboat Willie as a trademark and protect it through a different avenue, as time goes on they’ll inevitably lose their grip on the character.
Trademarks don’t have an expiration date - they can be used indefinitely, but they have to be actively used, otherwise they lose their protection. But Disney also can’t trademark all the iterstions on Mickey, so some of it will inevitably still seep into the public domain.
