One thing I don't like about these counter factuals is that they raise all sorts of issues about what variables you include or ignore. For example - as it was the attack pretty much did "destroy" the IV Corps but the two divisions of the III Corps were back from the point of attack and from Couch , so who knows how that would have played out. And what "command of the field" would have meant is an ambiguous proposition. FWIW, in his report Johnston stated only "Had Major-General Huger's division been in position and ready for action when those of Smith, Longstreet and Hill moved, I am satisfied that Keyes' corps would have been destroyed." It's also worth keeping in mind that the Federal left was a "defense in depth" in the sense of having successive division lines