I see no ethical problem in Cushing, who died in 1994, appearing anew as the evil Tarkin in Rogue One. Aside from his estate giving permission, what other permission was needed? Certainly not the general public's, whose permission would have been impossible to gauge anyway. In that way was Cushing or his memory disrespected by his inclusion in the new movie? To the contrary, I think his inclusion is testament to the power of his face, and the actor in this role and in general. If anything, the only valid complaint is that the digital configuration of Cushing as Tarkin in Rogue One was not that good - it was subtly too intense and out of place in the context of everything around it in the scenes in which he appeared. But still, all in all, I was glad to see it.
One could argue that Tarkin was not an absolutely essential character for Rogue One, which could have run just as well with other villains. But Carrie Fisher at the end as Princess Leia was indeed essential, as the literal embodiment of the word hope which she says.
Carrie Fisher of course gave her permission for the digital appearance as her younger self in Rogue One. And now that's she's gone, that last appearance will serve as wonderful parting word from Princess Leia and Carrie Fisher to the world.
Is that so different from Peter Cushing's estate giving permission for his character to appear again in a Star Wars movie? Rather than seeing what's wrong with this - because there isn't anything wrong with it, really - I prefer to see what's right, and thank the creators of Rogue One for bringing us the face of Peter Cushing and the grace of Carrie Fisher one last time.
See also Rogue One: Why Did They All Die? and The Force Awakens: Shakespearean and Fun and Ten Reasons to Like the Clones