Permission was given by Cushing's family and I assume Fisher gave permission for CG young Leia at the end of Rogue One. I didn't mind them in the movie, though it was clear to me how they were there. Honestly, if they continue to use these things, I'd prefer if they have the personal permission, and not just the estate's, for the actors' likeness before they do them, but it won't be a deal breaker, as long as the family grants it like now. However, I would also say, use it sparingly. If we have a movie full of CG'ed actors with the likeness of the deceased, it does cross over into bad taste. You can recast if you need to.

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tl;dr - A little is fine, but it goes a long way.