I think the reasoning for using the CGi Tarkin and Leia was that these events take place quite literally hours before we see then again in the original film. They should look basically the same as they did in 1977, or as close as possible. With Mon Mothma, her character wasn't introduced until Return of the Jedi, which takes place four years after these events. So she didn't have to look picture perfect, plus Genevieve O'Reilly, who played the younger version of the character still has a strong resemblance to the character 12 years later. Wayne Pygram unfortunately does not anymore resemble Tarkin without a lot of makeup.
This isn't a 1960s BBC production or a theater production were you can just get a different actor every week to play the same character as a matter of course. This is a multi-billion dollar film franchise that is functionally trying to make all its continuity work both in the storytelling and in the visual appearance of the universe. With the use of alternative shots from Red Leader and Gold Leader, as well as the CGi Tarkin, they went all out to make Rogue One as close to Star Wars as they could using today's technology. We will see how they handle Han Solo and Lando next year to see if they can break the mold that has been cast.
But, Rogue One and the Han Solo movies are not any of the three trilogies, nor do we have even the second film of the sequel trilogy out yet to judge. We are still four months away from The Last Jedi's release and Episode IX is still in the writing and preproduction stage. And for The Last Jedi....they have the real Carrie Fisher.