Now I honestly have no idea what the heck this whole MacGuffin bs is, but based on whatever I understand here, it seems to be about why Ash Greninja couldn't have just been Mega Greninja, as they are apparently interchangeable, and the story wouldn't have changed. However, that's completely untrue because of the Team Flare arc. Battles might not have changed, but the whole prophecy from Olympia was because of the special Ash Greninja form, that a regular Mega Greninja wouldn't have fulfilled.
Lysandre's whole obsession was on the Ash Greninja form, due to its special nature, despite Alain and MCX being stronger. Also, the whole point of breaking out of the Mega Evolution energy was because of Ash and Greninja being able to synchronize as one, which wouldn't have been possible if it was a regular old Mega Greninja. And lastly, Ash being able to see through Greninja to pin point Chespie inside the Megalith (and later in XYZ046) was only because of Battle Bond.
Now I agree to an extent that the whole Battle Bond could've been explained a little better, but I disagree that simply replacing Ash Greninja with a Mega Greninja wouldn't have affected the story. As seen with the Team Flare arc, the Bond Phenomenon and its effects were shown clearly, that a regular Mega Greninja wouldn't have been, and would have altered the whole crux of the story.