Again, the guy quoting the rules while the building burns down around them. You only have to actually read the Supreme Court's decision to realize that what the Constitution does or doesn't say doesn't matter to them, and by extension, doesn't matter when the person wielding that intentionally undefined power also controls the military. The law doesn't mean shit when there's a gun stuck in your face. Which plenty of people experienced first hand just a couple years ago, of the military marching through the streets and shooting at people at their windows, and randomly beating the shit out of them for existing.
But keep huffing the copium.
Well, yeah, I'm talking about the de jure situation because that's what matters to a conversation about the SCOTUS decision. The SCOTUS decision, btw, explicitly does limit the immunity to the President's Constitutional functions, it's not just magic dust and the President cannot make an act official by saying it is. To me, that suggests they do care about what the Constitution says.
But okay, let's talk about the de facto side in a hypothetical universe where the President doesn't have immunity.
The President decides to arrest Congress for whatever reason. Pretext doesn't matter, maybe he says it's sedition, maybe he says they're aliens.
President orders the FBI, SS, and military to arrest Congress, the leaders refuse.
President replaces the leaders with new leaders who will do what he says.
In the meantime, Congress probably doesn't like the order, if they know about it, and vote to impeach. Presumably, the President doesn't step down voluntarily so the Courts have to order the US Marshals to remand him into custody.
Here we have a divergence. Maybe the Courts are on Congress' side, maybe they're on the President's side. If they're on the Congress' side, they'll so order the Marshals; otherwise, they won't and there's nothing Congress can do.
In the hypothetical where they do order the Marshals to do so, the Marshals either refuse or they don't. If they refuse, who can stop them? If they don't, then perhaps the President fires the U.S. Attorney General, who is in charge of the Marshals, and replaces him with a political ally. Constitutionally, this appointment is void because the President has been impeached and is no longer the President (assume for the sake of argument that the VP is either on the side of Congress, or has also been impeached and the authority vested in the Speaker). But the question will really come down to which leader the Marshals decide to follow.
Meanwhile, the new leaders of the SS, FBI, and Army have ordered the soldiers to arrest Congress. Again, it comes down to who the soldiers on the ground decide to follow. The Courts can declare the order invalid and order them to stand down, but they cannot use mind control. The soldiers and agents will do what they choose. It will probably end up some level of civil war between them.
So what difference does the President being able or not being able to be prosecuted for it make? If the Courts are on the side of the President, then even if the President does not have immunity, they can just dismiss any charges for any reason they choose. There's no other recourse.
Complaining that I'm "quoting the rules while the building burns down" is missing the point. I'm specifically arguing against the idea that the Courts changed the rules and that's a disaster. If the rules don't matter, then the SCOTUS decision doesn't matter.