I just recently watched Leela’s departure, and the old quick, “I’ve grown up and fallen in love with a minor character in the last episode and have decided to leave” ploy didn’t really fly there, either. I think Louise Jameson wanted to be killed off and that’s understandable.
But I think you’re right, and although RTD I’m sure has other reasons for leaving, he has probably dried up on this. For me, Martha’s leaving was surprising (and disappointing!) but believable. I loved the way Rose originally left (I recall RTD saying that people hate the episode where their favorite character dies, and he didn’t want people to hat that episode). Now he’s borrowing from Charley’s departure (perhaps the one companion that foreshadows the new companion relationship) and I don’t mind that except he didn’t do it as well. But I do think Donna or Rose needed to die, otherwise what was that “most faithful companion will die” bit about?
The problem is, Caan’s prophesy undermined the suspense in that it meant we knew the other companions were “safe”. The only good reason for this is to build up to the death, and inevitably anything else is an anti-climax. Therefore, ultimately, it feels as though they might have improved the story a little by just cutting those lines (and the Ood’s ‘your song will end soon’ too) even without changing anything else. I will certainly watch the Donna stories again, but knowing what happens to her could have given the stories an extra dimension, if it had been done right.