I think that if Mangini left (and he could) that would really be the end of Dream Theater. One of the main reasons I feel that way, is because Mangini leaving would almost definitely lead to MP coming back....but it would be a complete disaster.
Fact: MP was (along with JP) the co-leader of the band.
Do you really think that MP will be given that same level of power and control within the band when he does return? I would say two things. 1) No freaking way does JP give 50% of the power back to MP and 2) NOT having the level of control he once did would stick in MP's craw like raspberry seed stuck in a denture.
And I'll add one more thing. The relationship between JLB and MP would be a sticking point. Before the split, MP was the co-leader. What he and JP decided on, was the way that things went. JLB had to go along with it. I think JLB breathed a big sigh of relief when MP left, and I'm not sure he would super excited to bring MP back. In fact, I'd be willing to conjecture that if MP did return, JLB would choose that time to "retire from DT under amicable terms" and maybe put more emphasis on his solo work and side projects.
TLDR version: the power dynamic has shifted so much since MPs return, that it can't go back to the way it was, and all the relationships would sour as a result.
Now. On the flip side of that. There is one possible positive outcome. As much as I have ALWAYS loved JLB, and felt that he was *the* voice of Dream Theater, I think we all admit that his voice is not what it once was, and that Dream Theater is a very demanding job. I think there is the slight possibility that everything that I just said happens (MP's return leads to JLB's departure) and the band find a young stud who can sing the old songs and breathes new life into the band. Someone with whom *everyone* gets along with, and the youthful energy re-invigorates the machine. But it's a very slim possibility.