I see your point, I agree that seeing the various other groups fighting back at least a little would have been satisfying, I cannot speak for the creators of course, but I think Dr. Wolf said it in his review that perhaps the reason was that they had so many other elements (Grogar, Sombra returning in the first place, getting the league of doom together, Twilight's new verb, and so forth) that they had to choose which elements to keep and which to dismiss, also as you mentioned they probably had to make sure that all the wittle kiddies wouldn't get bad dweams (I hope the sarcasm for PC culture is evident). Nevertheless, I think that if they keep with the momentum we can have a satisfying conclusion by seasons end and while I am a little sad by the fact that it is ending, I would rather it end on a high note rather than it just goes on for countless seasons to die off with a whimper.