He’ll quit or be fired when Trump turns on him.
At least for now, he’s doing all of Trump’s bidding and is the “fair-haired boy,” “teacher’s pet” in the cabinet But, that won’t last forever. Barr will do something, or refuse to do something, and he’ll land in the bad camp. Just ask a long list of former toadies – at best, their careers were damaged; at worst, they’re in prison or awaiting sentencing – how the ending goes.