I've heard an argument that a reason why Disney has pushed Steamboat Willie lately (new intro for Disney Animation films, and a lot of merch) is because copyright law works differently from trademark law. They can still claim a trademark even if the copyrighted work is in public domain. I'm not a lawyer, but if that's not all BS, I don't think we have to worry about anything like this anytime soon.
AFAIK you can only claim a trademark-violation if someone is (for example) selling stuff (so you couldn’t sell stuffed animals that look like an early Mickey for example).
A trademark just has to be "used in commerce as a mark". In layman's terms, that basically means distributing goods or services with it as a logo or a name. A stuffed animal could be infringement, but using something a logo for your software is much closer to the classic infringement fact pattern.