While you make a good point, only one director in all the years since the ten year term was put in place has served the ten years.
Many Presidents have asked for the resignation of the director and it has been delivered.
Trump is probably going to see what Sessions can get out of him and only make the change on his own time schedule. As you point out, this is not a cabinet office automatically changed at the start of a new administration.
I would like to see him gone to. Could be that Trump is waiting to get rid of later after Sessions gets in.
He is totally corrupt and has to go.