Well, it's not just the film, it's also everything in it, including (that design of) the characters themselves. When that enters the public domain, anyone can use (that design of) those characters for any purpose, including in their own works that have nothing to do with the Steamboat film. I can go make a platforming video game ala Cuphead using those characters and sell it. While I think that's a good thing for society, you can probably understand why Disney doesn't.
Consult a lawyer for exact details. there are things you can do with the characters after this expires, but the rules are very complex and I don't really understand them.
I kind of expect them to win that. But maybe they won't. Still, I wouldn't touch Mickey with anything less than the metaphorical ten foot pole and a really, really solidly constructed LLC or other corporate structure isolating it from any other asset I care about.
Note I am limiting this to just things they have clear trademark to. Grab the steamboat itself and do as you like. The soundtrack will be up for grabs. But I wouldn't expect to be able to defend myself in a trademark suit with the claim that the Mickey Mouse I used is not copyrighted; I expect the counterargument will basically "Yeah, but who cares? This is a trademark lawsuit".
Okay, yeah that's fair. Thinking strictly about copyright, I think what I said is true, but you're right there's other IP law at play here.
Given that Disney is infamous for its practice of rereleasing its older films periodically and otherwise making them completely unavailable (the "Disney Vault"), it seems that they have a business model which is fully predicated on copyright exclusivity.
Yes, obviously, there's someone buying it when they release it. But even in Marvel's current anemic state I doubt sales of Snow White reaches even .1% of the revenue from The Marvels, a single movie. Their revenue on direct sales of stuff about to go public domain is a rounding error, and nowhere near enough to justify locking up the entire rest of the nation's culture just for that.
Further evidence that it must not be that big a deal is that the Mouse seems to have finally relented and doesn't seem to be lobbying for more extensions anymore.
It's not about making money from Steamboat Willie, it's about preventing others from enjoying it without a monetary transaction happening. I'm sure if they could, Disney would rather destroy all copies of a work they weren't making money from than release them for free. Look at game companies fighting against people distributing abandonware. It's not about the value of that particular good--it's about the value of the other things they are trying to sell, and not letting customers get something for nothing.
"The works of the roots of the vines, of the trees, must be destroyed to keep up the price, and this is the saddest, bitterest thing of all. Carloads of oranges dumped on the ground. The people came for miles to take the fruit, but this could not be. How would they buy oranges at twenty cents a dozen if they could drive out and pick them up? And men with hoses squirt kerosene on the oranges, and they are angry at the crime, angry at the people who have come to take the fruit. A million people hungry, needing the fruit- and kerosene sprayed over the golden mountains. And the smell of rot fills the country."
Meanwhile, to keep this at-best modestly interesting historical film locked up for those last few marginal drops of IP, they've kept the entire rest of the culture locked up. Hell of a cost society pays just for that. I'd almost rather we just grant Disney copyright in perpetuity if it would shut them up and leave the rest of the culture alone.