Most defenders of originalism agree with Vermeule that originalism needs a justification outside of itself.
(For Part I, see here.)
Another argument made by Judge Kavanaugh is that eliminating prosecutorial discretion would involve the legislature exercising executive power:
The Framers saw the separation of the power to prosecute from the power to legislate as essential to preserving individual liberty. [Citing Montesquieu and Federalist No. 47] After enacting a statute, Congress may not mandate the prosecution of violators of that statute. Instead, the President’s prosecutorial discretion and pardon powers operate as an independent protection for individual citizens against the enforcement of oppressive laws that Congress may have passed (and still further protection comes from later review by an independent jury and Judiciary in those prosecutions brought by the Executive).
Once again, I think this argument does not follow. It is probably the case that
Congress cannot pass a law saying that the President must prosecute a particular person for violation of a crime. But that does not mean that Congress cannot pass a law saying that the President must bring an action against all persons where there is a probable cause to believe that they committed a particular crime. In this case, the President makes the decision as to whether there is probable cause, but there is no prosecutorial discretion. And if there are sufficient funds to finance all the actions, then the President must bring them.
It is easy to forget what the dispensing power was all about. One of the core cases involved James II’s decision not to enforce the law requiring a religious oath to be taken in order to hold certain offices, such as military command. Sure such a law would be unconstitutional in the U.S., but because of religious liberty, not because it takes power away from the executive. That the dispensing power in this case could have been seen as protecting individual rights did not make it legal.
The main argument that I have seen for prosecutorial discretion is that executives traditionally have been given this power. But that hardly makes it constitutionally required that they possess it. It makes sense to give the executive prosecutorial discretion because it is hard for the legislature to pass a sensible law that would regulate these matters. But that does not mean the legislature cannot take the discretion away in particular cases or constrain it various ways.