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
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.