To be honest, there are good arguments for both cases. For the record, Matt wouldn't be put back to his 'stutter' gimmick if he were to return. WWE seems to have moved slightly on from really, REALLY shit gimmicks.
However, we have to consider a couple of things.
1. Would Matt make it? I know that he is a very, VERY good talent. For a big guy, he is superb. But is he better than guys on WWE's roster? Whoere would you honestly think he would fit? Now, if Morgan was now a regular main eventer and former world champion in TNA, I would say that there is a strong chance, should he make the jump, that he would at the very least make it to WWE's upper midcard. But he isn't. He's main-evented a number of times. He's won the tag titles. Maybe there's a reason for that. And think, if TNA doesn't want to give him a world title run, would WWE?
2. Would WWE want him? If he did go back, where would they place him? Re-debut him like they did R-Truth (K-Kwik)? Reference his previous run and keep him the same guy, as with Christian and Jeff Hardy? Either way, he'd be in the midcard to begin with. So he'd be with guys like Ziggler, Ryder, Swagger, Miz, Barrett, Rhodes, Kofi, etc. Morgan's name in there would be a cool addition, but would he then be able to progress further, bearing in mind the amount of WWE-bred talent that is already there before him?
3. Has he missed the boat? I can't remember the thread or the poster that this point was made in, and I'm sure this has been brought up in here, but it's a point that I think is very key to Morgan's future progress. As I mentioned, Morgan has headlined a few PPV's and has beaten world champions, and has had title matches, but has never won it. Why? Does TNA not have faith in him? Or do they have faith until the last minute, and change their minds and go with the 'safer' bet of having either a champion retain or someone more 'established' win.
As much as I am a fan of Morgan, my belief is that if he was going to be both a world champion and a main palyer in TNA, it would have already happened. He may win the world title soon, and maybe it will ctapult Morgan to a new level, but I just get the impression that he could and should have achieved more by now. I won't say he's been wasted; look at his TNA run compared to his WWE run and you'll know that he's been used a million times better over on Impact. But that doesn't mean that he has been used to his full potential. He hasn't in my opinion.
But he's better off there than in WWE. He isn't like Christian or Jeff; a decent sized star when he left transitioned into a bigger star in TNA. He's been made in TNA. And we've yet to see a guy who has been solely established as a main player in TNA get a run in WWE to the top. And I don't think we're about to.
Either way, I think Morgan is too loyal to TNA for proving that he can be a serious player in a wrestling company. So whetehr it's the right thing for him to do or the wrong thing, my money is on that it will not happen.