Yeah, we should definitely fix the fact that the President can order hits on US citizens. That's a pretty obvious problem regardless of whether they can technically be prosecuted for it, and doesn't really change the merits of the question at hand.

All this case says is we shouldn't leave a President's legal culpability for any given action up to prosecutorial discretion. If they're using their official powers they're not culpable, if they're acting outside the bounds of their powers they should be prosecuted.

The actual problem is that the President has too much power, not that the next administration should have the right to prosecute them for exercising it.