Copyright was never meant to be perpetual, it nominally expires. The Walt Disney corporation has paid Congress to extend copyright essentially indefinitely to prevent works from falling into the public domain. Most importantly is "Steamboat Willie", the first work with Mickey Mouse in it.
As long as Congress continues to cash the checks that Walt Disney writes, copyright is an indefinite protection.
FWIW there's no talk of a copyright extension on the horizon and only a year and a half left to ram one through. The last major extension was lobbied for on the back of EU harmonization and free trade; that's been done and dusted. Few countries have supra-German copyright terms and none of them are dealmaker countries.
Furthermore, having the mouse fall out of copyright doesn't actually impact Disney's bottom line all that much, for two reasons:
1. They aren't losing much. You still can't put him on a t-shirt or sell any merch of him; that will infringe upon trademarks that will practically never expire. You are only limited to either direct reuploads of the public-domain material (which is practically worthless save for historical value) or including him as a character in another unrelated new work; and even then you will have to be very careful to only include aspects of him that were expressed in a public domain cartoon. Anything still under copyright casts a long shadow over the public domain[0].
2. The mouse isn't actually their flagship franchise anymore. Disney has since graduated to the FAANG strategy of "Can't innovate? Just buy your competition!". In this case that means buying up Marvel, Lucasfilm, and most of 20th Century FOX to gain access to more relevant - and, most importantly, newer works. Those works will not hit the public domain in any appreciable way for at least another half-century.
[0] My favorite example of this: Public Domain Sherlock Holmes is not allowed to have emotions because that's part of Still Copyrighted Sherlock Holmes.
Disney has been laying the legal groundwork for protecting the mouse for years now. Notice how all the new Disney movies have that little clip of the original mickey whistling. They are laying out a pattern so they can prove in court they are still using the imagery to distinguish their brand. Copyright expires but trademark is forever, as long as you are using it.
While I'm inclined to agree with you because everything you are saying is logical it doesn't pan out when I consider the history of it. If "Steamboat Willie" is not so important, then why were such herculean efforts pulled out to extend copyright?
I've lived basically my whole adult life dealing with this. Works never entering the public domain because of this copyright extension nonsense. Even college professors seem to think copyright is eternal. I had to argue with one that no, I do not need a textbook for a 19th century work. I can just read it on project Gutenberg or anywhere I want.
The EU wanted a single copyright term for their single market, so they picked the member state with the longest term (Germany), made that the EU-wide standard, and then put in a reciprocal extension clause basically designed to force America to go life+70.
At the time Disney was pushing hard into the European market[0] and having uniform copyright rules between two major trading blocks would be a huge boon to them. Keeping the mouse slightly more under their thumb than it could have been otherwise was just a bonus.
[0] Even when it was losing them billions of dollars, such as with the EuroDisney debacle
As long as Congress continues to cash the checks that Walt Disney writes, copyright is an indefinite protection.