Copyright is not like a patent where once it expires anyone can use the novel work.
Something else, while someone could repackage the earliest Mickey Mouse shorts once the copyrights expire, it is unlikely that someone could make new Mickey Mouse related stories, because of trademark and other issues - beyond that, 1923 mickey and 2017 mickey only have a passing resemblance.
http://createwebquest.com/sites/default/files/images/mickeym...
What we identify (as in most consumers of mickey related cartoons) identify as mickey mouse - didn't exist until fantasia in 1940. So long as WDC continues to make artwork with that character in it, and continues to keep up the trademark - the character itself will never leave copyright or trademark.