An apology for the Enabling Act that the Supreme Court has judicially inserted into American law goes like this:
By prosecuting a former President, Democrats opened a Pandora's Box under which every ex-President would be prosecuted by their rivals upon retirement, crippling the office, and discouraging high-quality candidates from seeking it. The Supreme Court had to prevent this.
Let's say this was your honest concern, and you are John Roberts.
You are not trying to render the Executive unaccountable. You want accountability, but some form of accountability that would deter routine political show trials of each party's highest-profile figure, prosecuted by the opposing party.
That might be a reasonable goal.
But if it really was your goal to deter "lawfare" while keeping the Executive Branch lawful, the decision you craft would look nothing like Trump v. United States.
Sure, you might grant pretty broad immunity to the President, as this decision does.
But you would not grant impenetrable deference to the President's pardon power. You would encourage, rather than prohibit, judicial inquiry into the motivation for pardons. Pardons for crimes ordered or encouraged or intended to serve the interest of the President would be invalid.
The President would still be immune. His subordinates, however, would not be. His suboordinates would still have reason to refuse unlawful orders. At the very least, they would demand legal cover before following deeply questionable commands. (Just as Obama did, before icing a US citizen in a drone strike.)
This is the accountabilty equilibrium we created after the Nixon Administration. Yes, Ford pardoned Nixon. Nixon got to live the rest of his days as a free man, even an eminence grise.
But many of his subordinates were imprisoned — H. R. Haldeman, John Ehrlichman, Charles Colson, John Mitchell, G. Gordon Liddy, James W. McCord Jr., John Dean, Dwight Chapin, Herbert Kalmbach.
Had Trump v. United States been the law in 1972, all of these men could have been peremptorily pardoned. No judicial inquiry could have been made into the motivations for those pardons. No criminal liability could attach to the President for making them. No subpeonas could penetrate a hermetic wall of silence, since any contempt charges could be pardoned as well.
Trump v. United States doesn't just grant the President immunity. It takes the heavily litigated notion of "executive privilege", renders it unjudiciable, and extends it as far and wide as the President wishes to extend it — with no remedy, no consideration of other interests, no hope ever of piercing it.
In its own words, Trump v. United States is about encouraging "bold", "unhesitating", "energetic", "vigorous" action by the President. John Roberts could have sought to balance the public's interest in an effective Executive with the public's interest that the President "take Care that the Laws be faithfully executed". He could have whispered into the Constitution the post-Richard-Nixon balance. He chose not to.
John Roberts and his henchmen are very familiar with this history. What they instead chose to whisper into the Constitution was a remedy for the accountability that the Nixon administration faced. They chose quite deliberately not merely to immunize the President, but to create an unaccountable Executive, whose breadth is as wide and encompassing as the President chooses by exercise of his power to pardon.