I agree with OP on a few of the points made. Especially the “not finishing the Dark Reign story”. Like only reading half a book or watching half a movie.
So what really got my attention was the part about “starting and not finishing”. It reminded me of my English Proffessor in high school. He was also the soccer coach and the volley ball coach. So when he had coach related things to do, he would put in a movie for the class, and sit in the corner at his desk doing his own thing. Then the next day, instead of finishing the movie, he would go back to teaching the class. Over the course of my senior year in high school, the class started 17 movies,(over half of which had absolutely nothing to do with the English class what so ever), but did not finish a single one. Basically, when my Proffessor had something better to do and wanted to distract the class so we would be quiet and leave him alone, he just popped in the first video he could find. Even though the entire class made constant jokes about this, it was still kind of irritating not being able to actually finish these movies, (a couple of them were even pretty good).
Not sure why, but this game, (and the OP here), make me think of that class for some reason. Hmmmm. Can’t imagine why. 
On a serious note, I don’t think MPQ has “Jumped the Shark” just yet. (For those unfamiliar with this phrase, it refers to the show Happy Days with The Fonz. After the episode where Fonzy jumped a shark on water skies, the once great show went down hill. So “jump the shark” means that something that was great has reached it’s peek and has started to decline). D3 has made a lot of improvements lately, and it’s an ever changing game. Not every change will be something the players like, i.e. “true healing” and shield cool downs. But overall, I’m very happy with the game.
Of course I would be much happier if D3 would finish up a few loose ends as stated in the OP. Like Drax, Starlord, Mr. Fantastic, Thing, Sabertooth, (seriously, 3 Wolverines and ZERO Sabertooth), and Prof. X. (again, all these X-Men, but no Xavier?). Not finishing what they already have starts a bad precedent. What happens if they do Civil War, or House of M? Will they just do half the story and move on to something else?