Creative incest this would have been.
Having the same director for both Trek and Wars is already enough. Do they also have to share the actors? That's silly.
There are 100,000 unemployed actors in Hollywood. Give other people a chance.
If they're right for the part and do a good job, who cares?
Cumberbatch is just the current hot commodity, which is Hollywood has a half-life of one or two years. Unless he really is "it" or has an excellent agent, he will go the way of Matthew McConaughey, Zach Galifianakis, and everyone else who was "the Shizz" and starred in every film for a six month stretch.