Rosenstein is running the DOJ. If he’s fired, Sessions will be a fish out of water; he has no idea how to step into the driver’s seat.
No matter what anyone says Sessions is a senile old politician who is used to talking about how great he is while doing nothing. Now he is so senile he doesn't talk anymore, he just keeps to himself.
He has no ability to actually be a real leader and run a McDonald's let alone the DOJ. He is a wormy politician, nothing more.