Yeh, he needs the grand jury not just for the employees that have left but for those who may be working outside the DOJ. Even though Rice, Clapper, Powers etc are gone, Horowitz would not have jurisdiction over them even if they were still there- and Durham will need to interview/squeeze some of their underlings who are career civil servants who are still there but over in non-doj functions. Durham could still interview them but he would not have any subpoena power without the grand jury.
Good opportunity to set perjury traps for all of them. You can do that just by requiring them to tell the truth. That will take almost all of the usual suspects down almost immediately. Then you can squeeze them from there. No reason why Mueller should have all the fun.