Despite the fact that the production team established a variety of different alternatives (such as recasting the roles or using holograms to hide the inadequacies of the CGI), the decision ultimately fell upon Kennedy to proceed and give both characters substantial roles in the film.
The team behind the movie had any and every opportunity to second guess whether or not the CGI resurrection process would work.
John Knoll recently spoke to io9 about the process of recreating Grand Moff Tarkin and Princess Leia for Rogue One: A Star Wars Story, and admitted that Kathleen Kennedy was the one who remained resolute about this particular creative decision throughout the film’s production.