Sales going down by someone using your original world/characters in a way that is then deemed unpopular is definitely restricted/not possible in a world with enforceable Copyright.

And that was my point, not sales going down on their own.

Negative reviews have nothing to do with someone else being able to take e.g. James Bond and making him a homosexual or taking Conan the Barbarian and making him a woman or a sidekick etc.

Drastic or even small changes can always be unpopular and affect the whole franchise. If said franchise is no longer under your control alone, this is an argument pro copyright from the viewpoint of a creator.