There is a precedent, and you alluded to it.  So, if Rosenstein had made his case, and Trump had agreed, Trump would have to expend political currency to have it passed as an act of Congress. Given the sanctification Mueller already had received, he was a hard to decline candidate.  Why would the situation call for such an extraordinary situation? They already knew the shit ran deep. Who is who might have been a different matter.  It would have not only expended Potus political change, it also would have cemented Mueller in place, and then, Rosenstein and Mueller could play a nice 1-2.  Potus did not buy it.  So, next day the second best option was executed: special counsel.  Comey was the bishop in the game that could easily be "sacrificed" with the cover in place.  It was a sacrifice that did cost nothing.  And that is exactly the M.O. of the holy trinity: RCM.  Remember their heroic stance on the nsa surveillance? Yeah, nothing was done. Only bukake.  A stance that cost them nothing. It is easy to be a patriot at such high expense.  Now that the blockade is removed: the heat was immediately turned up on Comey.