Even if they fail to extend the original copyrights, there are other potential "loopholes" to making it harder to reuse these characters:
The more Disney uses variations of them, the bigger the minefield becomes in terms of sticking to the depictions that enters/have entered public domain. And they may also be going for making it harder to make any use of some of the imagery without running into trademark law as well.
In this case, the author "only" had to deal with the threats and the small minefield of avoiding character aspects revealed in the last 10 Sherlock stories, and so presumably had a reasonably simple task. Perhaps the Arthur Conan Doyle estate now wishes they'd pursued a more aggressive licensing strategy aimed at creating a bigger minefield..