But again, that isn't how it works. The minute they attempt it they would be sued by whatever actor they attempted to replicate without permission, whether prosthetic, makeup, cgi, or deepfake. Going back to the Crispin Glover lawsuit, Universal attempted to argue that the person in the movie was "George McFly" and not Crispin Glover, and thus attempting to replicate Crispin's face was perfectly fine. It never stuck, thus why Universal ultimately settled with Crispin for 700k dollars (his lawsuit asked for a million).I really do think it's because if the studios go all-in on deepfakes like you suggest, the message that sends (real or imagined) is, "Henceforth we no longer need real live actors."
The fact actors are even more anal about "likeness rights" due to things like videos games and animation means it's unlikely a studio would be able to get away with it.