Let me clarify, only the Steamboat Willie version of Mickey Mouse characters are in public domain, not the recent versions as Disney holds the copyright. In a few years, more Mickey Mouse shorts will become public domain.
Personally, I don't give a shit about whether Mickey Mouse is copyrighted or not. What I care about is all the other works that were kept from entering the public domain because Disney was constantly getting copyright extended.
What's funny is that Disney built their empire largely on public-domain works (such as fairy tales), but when it's their turn to give back, they fight it tooth and nail. Classic getting to the top and then pulling up the ladder behind you.
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.