But JJ Abrams is the one guy who just, all of sudden, definitively 'proved' that recasting is 'crap'?
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Yep.

But to be fair we have to narrow the field a bit. Superheros get recast all the time. Characters there are also one off recasts (generally in pilots) like Father Mulcahy and Saviik, or occasionally seen characters. Plus you also have to consider the limitations of the time. We are just now entering into an age where we can recreate the look of original actors. Before you really only had two options, recast, or remove the character. So recasting was really more of a necessary evil. Dr. Who is interesting in that it incorporated recasting into the character.
This also ties in with remakes. As a general rule I dislike remakes in visual media(except for the previously mentioned superheros as that appears to be an essential part of the genre.). That's mainly becasue I feel it dishonors the original work. Who has gone and started watching the original Hawaii Five-O because they watched the remake series? A handful? But if they had made the new series a continuation/sequel to the original then I bet far more people would watch the original, becasue the new series would not be a replacement for the original.
The reason why I singled out JJTrek is becasue is both recast classic characters whose appearance has been established for generations; and becasue as a reboot it is also a replacement for the original show. The Hawaii Five-O reboot is crap. The MacGyver reboot is crap. Even if the writing on the show is amazing, it's still crap just becasue it is a reboot.
You can recreate the character, you can create a facsimile of that person and their nuances. But at the end of the day, it's still not Peter Cushing. It isn't what he brought to the role, his presence on the screen, his training as an actor. And it never can be. At least when a role is recast completely you can get a sense of a new actor paying homage to the past while bringing their own interpretation to it; CGI and an impersonator can only ever be a walking meat-puppet version of the man.