Interesting that the Grassley letter also calls for the deputy director to also resign.
Normally it’s the deputy director who takes charge when the FBI director is removed, so would run the agency in the interim?
There is a line of succession. Government agencies run, for the most part, on automatic pilot. The permanent bureaucracy or administrative state doesn’t depend on political appointees to function. It is why the Deep State is so pernicious and dangerous.
The CIA has only three political appointees - the Director, Deputy Director, and the head of the IG. The FBI is not much better. Google Plum Book and you can view all the political appointee jobs by agency.