when the Steamboat Willy copyright expires ...
Someone could make a derivative work of the Disney short and call the title character something other than Mickey Mouse, but if they tried to call him Mickey Mouse, Disney could take action against them for using their trademark without permission.
Wrong, because the instant the copyright expires, a large part of the trademark lapses. Disney no longer has any rights to base a cause of action on. The reason for this is that because copyright law no longer prevents anyone from making copies or derivatives and putting them into commerce, the MICKEY MOUSE mark changes from being descriptive (of the character named Mickey Mouse) with secondary meaning (which can only originate from Disney) to being merely descriptive, without secondary meaning.
It looks to me as though you're putting the cart before the horse, incorrectly believing that the trademark survives the entry of the work into the public domain. But it does not; only a few fragments of the trademark survive.
Ultimately, copyright and Trademark are two wholly different pieces of intellectual property that govern entirely different things, handled almost entirely orthogonally to eachother, and in practically all cases, one has absolutely no effect on the other.
Sure. But this is one of those exceptional cases.
The only way that trademark could possibly be affected by Steamboat Willie falling into public domain is if that would or might cause the public to not realize who Mickey Mouse belongs to, but since the copied work is still a *COPY* of the work, and so would still be clear who owns the intellectual property that is still very much alive.
Actually that is exactly what happens; when everyone and his dog can legally create new, derivative, Mickey Mouse cartoons, because the underlying copyright has expired, the public is assumed to no longer associate the MICKEY MOUSE mark, to the extent it pertains to goods including creative works, only with such goods made by Disney. This is SHREDDED WHEAT, which I'll get to in a moment.
Also, note that trademark doesn't care about whether a work is a reproduction of something or not. It cares about the origin of a specific, tangible copy, not of the underlying work. If you start Mark-T Press, and print up copies of Romeo and Juliet, I am not allowed to start Kangarooski Press and print up copies that bear your mark. OTOH, I am perfectly entitled to print up copies of Romeo and Juliet under my own mark. In fact, so long as you're just reprinting the play (and not making such substantial changes as to amount to a new work, which is a bit more difficult than you'd think), I'm even entitled to make copies of your version, so long as I take care to not use your mark and to only use my own, thanks to Dastar, which eliminated reverse passing-off for works (and hopefully is the beginning of a trend of eliminating reverse passing-off altogether).
Be aware that the copyright status on the short has already expired in several countries that do not practice the copyright durations the US currently has in place, and the cartoon can be freely distributed or copied in said jurisdictions, while the trademark status has remained entirely unaffected.
I'm only familiar with US law, and that's all I've been discussing the entire time. I have no knowledge or interest in how other countries handle this, except as how it might provide us with good ideas or cautionary examples in our own legal reform efforts.
I am not directly familiar with the Shredded Wheat case you mentioned
It is Kellogg Co. v. National Biscuit Co., 305 U.S. 111 (1938).
The relevant language:
The plaintiff [Nabisco] has no exclusive right to the use of the term 'Shredded Wheat' as a trade name. For that is the generic term of the article, which describes it with a fair degree of accuracy; and is the term by which the biscuit in pillow-shaped form is generally known by the public. Since the term is generic, the original maker of the product acquired no exclusive right to use it. As [defendant] Kellogg Company had the right to make the article, it had, also, the right to use the term by which the public knows it. ...
Moreover, the name 'Shredded Wheat', as well as the product, the process and the machinery employed in making it, has been dedicated to the public. ... Since during the life of the patents 'Shredded Wheat' was the general designation of the patented product, there passed to the public upon the expiration of the patent, not only the right to make the article as it was made during the patent period, but also the right to apply thereto the name by which it had become known. ...
It is contended that the plaintiff has the exclusive right to the name 'Shredded Wheat', because those words acquired the 'secondary meaning' of shredded wheat made at Niagara Falls by the plaintiff's predecessor. There is no basis here for applying the doctrine of secondary meaning. The evidence shows only that due to the long period in which the plaintiff or its predecessor was the only manufacturer of the product, many people have come to associate the product, and as a consequence the name by which the product is generally known, with the plaintiff's factory at Niagara Falls. But to establish a trade name in the term 'shredded wheat' the plaintiff must show more than a subordinate meaning which applies to it. It must show that the primary significance of the term in the minds of the consuming public is not the product but the producer. This it has not done. The showing which it has made does not entitle it to the exclusive use of the term shredded wheat but merely entitles it to require that the defendant use reasonable care to inform the public of the source of its product.
the terminology accurately described the product in the first place
Are you suggesting that MICKEY MOUSE does not 'accurately describe' a product including the Mickey Mouse character?
As a counter-example, while Lego's last standing patent on Lego bricks expired in 1989, nobody else that makes so-called compatible building blocks is allowed to call their product Lego, nor are they allowed to even explicitly say they are compatible with Lego (even if they are) without first getting permission from Lego to do so (and they would be extremely unlikely to receive such permission, since they would be a direct competitor), because Lego still holds trademark status on the term in the context of a toy (and also as a company name).
As the last part of the quote from SHREDDED WHEAT discusses, there is a difference between a mark that is associated with a product, and a mark that is associated with the source of a product. If you sell ELEVATOR-brand vertical conveyance devices, you're doing okay. But if the public associates the ELEVATOR mark with the actual device itself, then you lose your trademark because it has become a generic term for any such device. This is genericide. It's happened with ELEVATOR, TRAMPOLINE, ESCALATOR, THERMOS, and YO-YO. It came very close to happening with SANKA but then people started using decaf as a generic term instead. XEROX, BAND-AID, VELCRO, KLEENEX, Q-TIP, and in fact, LEGO have been teetering on the precipice for years. But this is why you see their advertising very clearly refer to things like VELCRO-brand hook and loop fasteners, KLEENEX-brand tissues, and in the case of legos, LEGO bricks. Xerox has been running public awareness ads for decades in order to keep their mark alive. My favorite had the tagline 'You can't xerox a xerox on the xerox,' pointing out that they would greatly prefer it if people did not use the XEROX mark as a generic term for photocopying, photocopies, and photocopiers, respectively.
If the applicable patents are expired, then anyone can make LEGO compatible bricks. And thanks to the nominative use doctrine of trademark law, anyone who does make LEGO compatible bricks is allowed to say that they are compatible with LEGO bricks, so long as they don't misrepresent their bricks as originating from the Lego company itself.
But Lego NEVER refers to their product as LEGOs. That would be to invite the loss of the LEGO mark. They refer to their product as bricks. Go ahead and tell me that MICKEY MOUSE doesn't mean the Mickey Mouse character, in the minds of the relevant portion of the public.
Of course, if Disney had trademarked the title "Steamboat Willie", they would lose certainly lose trademark protection on that title once the work with that fell into public domain.
You can't trademark the title of a creative work for the same reason; it's a descriptive term for the work itself, without secondary meaning. When you see a trademark in a title, it's because it's part of a series of similarly named works. Like for Harry Potter books, the HARRY POTTER mark can only arise because all the books are named Harry Potter and the something something. If the titles were all totally distinct (e.g. "The Sorcerer's Stone," "The Chamber of Secrets," etc.) without an overarching series name, the titles would not be trademarkable.