Yah. That's hyperbole--I'm sure we can all come up with examples of why an author (even one who isn't Conan Doyle) would kill a character otherwise, even a protagonist. But for the most part, I think most authors really, truly feel that they are doing what the character's arc requires.
So how do we talk about where they--we--screw up? What are the markers for where the author has generally failed to convey the intended arc to the reader, or where the readership is going to overwhelmingly disagree about whether it fits? I think that's a far more contentious question, because unlike the generality, it does better if we point at specifics--not just specifics that worked, but specifics that didn't. And I wonder if we end up talking about TV and movies in this kind of conversation not just because there's less to watch than to read and therefore more assumed overlap but also because we have less risk of having to point at a friend or colleague and say, "You. You really screwed that one up."
The thing is, we often get to this sort of point on panel discussions. Someone will say something like, "The story needs the ending it needs." Which is absolutely true, and a lot more satisfying than alternatives such as, "The story needs the ending dictated by convention." Sometimes that first-order generality needs saying. But I think it's much harder to really dig into what that means and what it doesn't, and sometimes we're so thrilled to have found agreement on a panel that pushing past it back to disagreement sort of gets lost. I'm not sure, other than disagreeing really politely and kindly and giving each other the benefit of the doubt, how to make that happen less often on panels. But it seems like it might get to some worthwhile insights, even if they don't apply for everyone's process or everyone's specific story.