First off, I don't even think you'd be obligated to follow all living guys' wishes.
Second, dead people are dead. Nothing can ever do them any harm. You might argue that it makes *us* worse off to not follow a dead guy's wishes in some way-- you could argue that it cheapens society. But, in this case, I'd argue that the benefit outweighs the harm.
I have a lot of respect for the Burkean argument that the social contact is not just between the living, but between the living, dead, and yet unborn. (See Philosophy, Vol. 63, No. 243 (Jan., 1988), pp. 111-113.) It's just that in this case, the benefit to society of having another great literary work by Nabokov outweighs all other considerations.