SilverWook said:

It's been done more often than most people realize.

https://www.youtube.com/watch?v=YEgsqvtaN28

Pretty sure it's always done with the permission of an actor's estate or family. Tarkin may not even have a large role in the film, or only appear on a viewscreen or hologram.

I'm not super comfortable with a lot of those either. Honestly, based on what I know of Cushing and how peripheral the role is likely to be, I don't have a huge problem with this idea in itself, but it seems like a very slippery slope. I know copyright law is good enough to prevent these particular examples, but I've got a horror scenario in my mind where we end up getting Godfather prequels with "Brando" and Casablanca sequels with "Bogart". The realist in me figures this is ludicrously improbable for myriad reasons, but the pessimist in me can't shake the feeling that if this is established as a viable possibility and the powers that be like what they see when they run the numbers, something in that vein could become our reality. It's much better if everyone agrees that it's a dick move now and proceeds with pursuing other methods, IMO.