If JEJ granted voice rights, then that’s peachy. Nobody owes someone else that job. Sounds like JEJ locked it down.
The idea that actors are required to be hired is kind of nonsense. Every industry has had to deal with less people required to produce the same amount of output. At this point, JEJ voice of Darth Vader IS Darth Vader. I can understand the studio wanting to retain that. Replacing voice actors is often jarring for the viewer.
As is their right. His estate, or him personally, doesn’t matter.