I think you're right. Mark Gatiss has a fantastic career, he can write, play dramatic and comedic roles, on camera and on stage, he has directed movies and written novels. I can't see him sacrificing all that for a stint as Doctor Who showrunner.Gatiss seems to flit between projects, I can't see him as a showrunner, devoting an awful lot of years to it as Moffat has done.
I think we'll end up with a dull BBC apparatchik, one that will run Doctor Who as if it were just any other show.