Here's a 2020 Comic-Con panel that talks about the legal intricacies of using famous public domain content.
https://www.youtube.com/watch?v=YX6H7Q0v-UkThe reason I don't think we'll see copyright extensions in the foreseeable future is that trademark protections last forever.
Copyright laws preceded trademark laws by over a century and don't work well together when it comes to intellectual property. At their core, copyright protects creative works for a specified period which is currently the life of the creator and a defined period after that while trademarks protect brands forever. Copyrights protect creators while trademarks protect companies. Successful companies build a reputation based on the quality of their products. They don't want that reputation destroyed by a company producing cheap, inferior knockoffs using their name.
Ever buy a no-name Lego compatible brick set that didn't assemble as nicely as a Lego set? Lego doesn't want any consumer thinking that knockoff is as good as what they make. They're so protective of their name that the knockoffs can't even say they're compatible with Lego. All that can be said is that they're compatible with leading brick systems. That's the kind of protection that trademark law works best with. There is no conflict with copyright law for this protection.
Problems arise when trademarks are used to protect companies built on intellectual property where there the protection is on creative ideas rather than physical product. It's a type of industry that I doubt the original crafters of trademark law gave much thought about. Oddly, intellectual sweatshop types of industries are a relatively modern problem where companies own creative works rather than creators. The book industry has thus far mostly resisted that temptation and stuck to its traditional origin as being the distributor of creative work rather than the owner. In the music, movie, and comic book industries, creators often own little, if anything, of what they create.
Part of the issue with books versus other creative industries lies in how many individuals are involved in the creative process. Typically, a novel is a one-person creation. A movie involves hundreds of people. A song writer usually isn't a singer. A singer isn't always a composer. Singers don't play all the instruments on their recordings. The more people that are involved in a creative work, the more difficult it is to determine who deserves most of the credit for the final product. For example, take the Stone Poneys hit song, "Different Drum." Mike Nesmith, of Monkees fame, wrote the song as a countryish ballad originally sung by the Greenbriar Boys. It wasn't a hit. With a different arrangement and lead singer, Linda Ronstadt, it was a hit. Who gets the lion's share of the credit for the song's success?--the songwriter, the arranger, or the singer? It shouldn't be the record label, but it might.
As far as beating Disney by a million people releasing new animated Mickey Mouse, cartoons, that's not going to happen. First of all, there aren't a million people with the talent and time to do that. More importantly, Mickey is the legal trademark of the company that was originally founded by the man who created (with Ub Iwerks) Mickey. Legally, anybody creating new Mickey Mouse works will be successfully sued and put out of business by Disney.
Steamboat Willie will enter public domain in 2024. It will be hard to release it on disk without being sued by Disney. By not using Mickey's name or image, a company might get away with it. Theoretically, somebody could produce a comic, cartoon, or toys of Mickey in the original art style of the public domain version of the mouse. They won't be able to call it Mickey Mouse because that name is too strongly associated with the Disney brand. With those restrictions, why bother? Parents aren't going to buy their kids a Mickey Mouse toy that doesn't look like today's Mickey and isn't called Mickey Mouse.
Let's not forget the lesson of Warner getting websites to remove most of the Captain Marvel Adventures comics. Not everybody wants to mount a legal defense against a giant corporation. If YouTube puts a dollar rental of Steamboat Willie up, Disney's threat of a lawsuit isn't going to legally scare a company as big as they are. A lawsuit against Google risks setting a legal precedent against Disney's trademark protection that would render legal suits against smaller companies toothless.
In another decade, Superman and Batman enter public domain as well as many classic talkie movies. Legal challenges will eventually occur that better define the limits of trademark protection.
File sharing of public domain content, including Steamboat Willie, won't be a big concern for giant holders of intellectual property trademarks. Their bigger headache is stopping current products from being disseminated without their control. The amount of revenue lost from Steamboat Willie is minuscule next to pirating of new releases.