Trademarks don't expire so long as they are being used.
Yes, I know.
That's what I meant by leveraging Trademarks (which do not expire unless they fall into misuse, as you said) to get around the expiring Copyright terms of some key Disney material that will happen in the next decade.
In short, Trademarks were never meant to be used the way in which they are today. They were for unique company names, logos, etc. - no one envisioned at the time that multi-purpose characters, etc. would end up being Trademarked literally up the wazoo (individual character elements, etc). They were never intended to cross with Copyrighted material, the same thing wasn't meant to be both Copyrightable and Trademarkable.
Because of the various extensions, this issue keeps getting pushed back - and after the last extensions, you saw companies like Disney filing an absurd amount of Trademarks in preparation for this day. It's ridiculous, there are something like 100+ Trademarks on various parts of Mickey himself alone.
The recent various Superman/Superboy cases brought up a key question that is just the start as Disney and other companies finally start to lose their copyrights (there is no way politically they will be extended again) - "What constitutes a character?" The answer is complex, but essentially it comes down to - a character is the unique qualities they display in the work.
So, although the claimants lost the various Superman cases eventually (due to upholding of previous agreements made by heirs over the years), there was a point where it looked like Superman was going to be broken up - the basic character that could leap and parts of the costume shown in the first issues could have ended up with the family, and then DC would retain ownership of things added later, like laser vision, etc. Basically, it would have been a mess and no one could have used him unless they worked together.
For Disney, an example would be Steamboat Willie. Steamboat Willie introduced the character of Mickey Mouse, and when that film enters the public domain, as the system was designed, so would Mickey - as he appeared there. So since Pluto didn't show up till a few years later, Mickey couldn't have a dog until Pluto's first appearance became public domain. But...Mickey (and Pluto, and everyone else) is now Trademarked up the wazoo. So that severely limits the intent of expiring copyrights (past the absurdity of how long they were able to extend them to begin with).
TL;DR? Disney is going to lose key Copyrights in the coming years, and their only legal weapon to combat it is going to be the ever-living Trademarks, which depending on how courts legally interpret things, could be used to get around the original intention of expiring Copyrights and Public Domain. It's something that is going to be a very big deal, not just for Disney, over the next decade or two.