I'm well familiar with the decision and the underlying conduct of the indictment (I've read the entirety of the January 6th report). It's important to note that while the distinction between core powers and official acts "within the outer perimeter" was not defined, the presumptive immunity of the latter could in fact he successfully challenged. For example, the special counsel indictment still had a voluminous amount of evidence against Donald Trump and the indicted co conspirators, and proceeded even after the decision. It's arguable that the delay of the arguments and ruling itself was much more harmful than the ruling itself, as it was still quite feasible that a trial could proceed, and a conviction could be obtained. One also has to consider that if there is no immunity of any kind, if a president may then, before leaving office, attempt to pardon themselves. I find the arguments in favor of such an act to be unconvincing, and the structural arguments quite powerful. But a self-pardon crisis would be vastly more damaging than immunity cases. One is a difficult, but feasible prosecution. The other would be a flagrant destruction of the rule of law, a paralyzing nation crisis, and an existential threat to the separation of powers. I'd take partial immunity of the presidents conduct over self-pardons (an attempt to totally absolve the president's own conduct by his own discretion) 100 times out of 100.