You can't just extend copyright indefinitely. It's not like a patent, where you can make minute changes and claim it's a new product. The original works have a copyright limit of 95 years after the first date of publish (thanks Disney and other corporate lobbyists).

If we go by The Return of the King, it was published in 1955. That means the words, the story, the settings, and the characters will be public domain in 2050. Steamboat Willie, on the other hand, was published in 1928. That means it expires at the end of this year. Unless Disney can convince Congress to change copyright law again, these copyrights all have hard expiration dates.

ETA: Disney might have a case where they can claim copyright on the information they added or changed from the original works, just like how they can still claim copyright over Mickey after losing Steamboat Willie.

And I'm sure they will, because fuck society, amirite? /s