Still I have little sympathy - it's hard to argue that endless Twilight fan-fiction did the franchise any harm. And there's plenty of non-canon James Bond films that have no impact on the main franchise.
I definitely support a short copyright period (say 10-20 years). Beyond that, as the original creators of the characters, any further 'official' content/stories created by them are likely to be popular if they are any good (and would be subject to their own copyright period). But if someone else takes on those characters and manages to produce great stories with them. Well fantastic, let's allow them to, and reward them for it.
I am bitter that Zelazny's heirs killed Amber RPG, one of the best of the time.
A lot has to be said on Dune and LOTR heirs as well. And don't get me started on Tintin, currently managed by the surviving second husband of Hergé's wife...