You guys are operating under the assumption that Marvel digitally replaces their heads.
The process is actually more like a moving photoshop job where they’re manipulating the actor’s actual faces instead of creating digital doubles.
This process just wasn’t possible for Rogue One because the key actor is deceased. As for Carrie, how to put this delicately, she would not have been able to double for her 19 year old self.
Actors with the same build and facial structure?
CG Tarkin was great in the first viewing. CG Leia worked but a little distracting. In the second viewing they were both a bit distracting.