It is essential to have Mueller be impartial. He needed a fair shot at POTUS investigation. POTUS knows there's nothing to find.
If Mueller was /ourguy then the investigation is a sham which makes us no better than them, we'd also be weaponizing the DOJ for our own ends. We probably only have one caveat, we get to decide when to pull the plug once it's confirmed nothing is/was found. That is now happening with Rudy.
Great Movie.