This is quite a shock to me as well. I've always equated Dream Theater with Mike and JP. Without Mike, the band won't be the same. This might be good for the band, it might be bad for the band, and it might destroy the band. Also, this could be great for Mike or it might be a bad decision down the road.
Quite honestly, I thought that the last couple Dream Theater albums have been catering to a different crowd than they used to. I'm not talking going back to Images and Words...I think if you listen to each album from Majesty to Train of Thought, you see that the band morphs from one sound to another quite consistently. I didn't purchase the latest album, and it's the only one I do not own, particularly because it just didn't grab me at all.
So maybe this will provide both parties with some room for growth.
Also, if Dream Theater did break up, (which I could see happening without Mike) I honestly think that it'd be a rocky time for the guys, but that they would continue to prosper in the long run. I think that John Myung has a huge reputation and would easily get plenty of offers. Obviously, JP is very well known and could join any one of several bands. Jordan might be the toughest one to place, but he's also most adaptable. And James LaBrie could spend more time promoting his solo career.
I could see all of this going quite well, in the best case scenario, or terribly wrong...either way I'm nervous but also excited to see if this ends up being a good thing.