That's just if somebody managed to kill everybody all at once above each person. For example, they would have to kill the top six to get to Mattis. (Probably at least competent, since he is a General.)
A much more likely scenario is that Trump goes and we get President Pence. Then he fires everybody and replaces them with his own guys. It is somewhat rare for a new President to keep appointees over from the last administration unless they have longer appointments like the head of the FBI.
Pence would probably be more competent than Trump. (It would be hard to be worse.) He would most likely make some conservative but competent appointments. We might not like them, but his administration would at least be competent and not Russian agents.
