The relevant case here is a recent decision regarding the Sherlock Holmes books published before 1923. The characters represented in those works (i.e. without later developments in still-copyrighted books) are in the public domain, and can be freely used in new stories. There's no reason to think that the same wouldn't apply to Micky and Donald.
There are also plenty of examples where new (and valuable) IP is still being created from material that no one (not even heirs) disputes is in the public domain.
How many remakes of Shakespeare, Jane Austen, heck, stories from Greek mythology and the Bible have been made? Lots.
As many have noted (including someone upthread), Disney itself strip-mined the public domain for many of its big-budget pictures.
http://artsbeat.blogs.nytimes.com/2013/12/27/sherlock-holmes...