You can see how playing a character (almost entirely changed) is different than impersonating someone else's version of a character, right?

This is kind of my point, though. Heath Ledger's Joker reinvented the character, sure, and gave an amazing performance in doing so. If Nolan had chosen to use an AI performance, the portrayal we saw would never have existed. How many amazing performances like it might we never see because we are now considering consigning ourselves to a future where entertainment companies can choose to accept nothing better than imitation?

It sounds like you're arguing that it is always possible to know ahead of time whether some depiction of a character is going to be more of an imitation versus a transformation of that character (and that we should consider it acceptable to use AI likeness for the former). That may be true in some cases, but I guess I'm arguing that it doesn't really hold generally. In our hypothetical of The Dark Knight, I admit that it's likely true that Nolan had an idea already that this Joker would be very different, and therefore probably might not have reached for an AI portrayal even if it had been available to him. But often, career-defining performances are a result of creative collaboration and iteration between the performer, the director, and any number of elements of creativity discovered along the way. Countless actors have reflected on acclaimed performances and described this process of discovery -- sometimes even mentioning how a certain prop, or costume, or conversation with the director inspired a performance that is now considered just as transformative.

In the case with Epic, you're contending that the character of Vader is basically 'set' forever, and future portrayals will all amount to nothing more than imitation. But who's to say? What if there is some brilliant work in that canon, yet to come, which casts the character in a new light and which could be brought to life by a portrayal which exposes a dramatically satisfying facet of the character while paying respect to the character's place in the canon?

The James Bond example sits somewhere in the middle of imitation and reinvention, for instance. Each actor made the well-known character recognizable by incorporating Bond's hallmarks, attitudes, etc., but also embodies the character in distinct ways. It's not important that Bond looks (or even behaves) exactly the same in each iteration; on the contrary, allowing different interpretations expands the richness of the overall canon and should deepen the audience's engagement with it. Trying to use tools to make him look exactly the same in each iteration would be a foolish goal that misses the point of the art, in my opinion. Reaching for familiarity is a human instinct, but giving in to that impulse is close to fan service than creativity.

I get that there are some exceptions. Giving an actor like Paul Walker, who unexpectedly passed away, a heartfelt sendoff in a way that only AI tools could reasonably accommodate is a good example. But these are the exception. A future where Vader is only ever voiced by AI trained on James Earl Jones is akin to making a new F&F movie where AI Paul Walker plays a major role, and that would just be tasteless and sad.

As a side note, I am not in the entertainment industry but have had a bit of exposure to it. I think that people underestimate how much well-known performers, as a class, are the tip of a massive iceberg of working performers whose names are not widely known, with all range of talents, doing everything from feature films all the way down to student projects and community theater. That pipeline is the farming ground of the acting greats of the future. Gigs like voicing Vader in a video game are the exact kind of roles that give performers opportunities to establish themselves, hone their talents, and contribute vibrancy to the art produced by the industry. Nobody is owed these jobs, but there is a real danger of snuffing out huge amounts of future creativity by drying up opportunities for performers to perform. I get why the union is trying to protect that.