James seems very happy in DT, always has, and The Astonishing was basically his crowning achievement. Will he have to adjust as he ages? Sure. They all will. Does that mean he'll leave or be replaced? I find it highly unlikely. If he hasn't left or been replaced by now, I doubt he will in the future, unless due to health reasons or family reasons. I certainly don't think it'll be due to the simple fact that he can no longer sing the way he did twenty years ago. It isn't at all difficult to think that he'll be in the band until they retire, especially if things continue to sail smoothly.
Besides, can any of you really picture JP saying "James, we're family, we've been thru everything together, but since you can no longer sing the high over-the-top parts from our older material, we're bringing in this other guy who is way younger, and can get the job done." Seriously? I can't. That's like telling your wife "Hey babe, I know we've been thru a lot together, but...you really aren't hot anymore, so, I'm going to start banging this other chick, but don't worry - we can still be friends." I just don't see this kind of scenario happening.
Honestly, if any of them were to leave at this point, I think the most likely would be JR or MM. And, between those two, I'd say MM. You can tell that it's basically just a job for him. I'm not saying he doesn't enjoy it or take it seriously, but compared to the others, he hasn't spent over half a lifetime cultivating this band, turning his vision into a reality. Of all of them, he's the guy who has invested the least amount of time and effort. Dream Theater is obviously much more special and meaningful to JP, JLB, and JM than it is for Mangini. So, for that reason, I definitely wouldn't be surprised (at all) if he were to call it quits after a few more albums/tours; maybe even sooner.
Jordan for the most part seems to enjoy being in DT, but is hard to read sometimes. The problem is, he's such a monster player, and he doesn't really get to use or explore his full potential in DT. His joining the band certainly took them to a whole new level, but since then, he has still had to work within certain set parameters. For this reason, and also due to his stern hard-to-read nature, I've often wondered if he really is happy in DT. I definitely wouldn't be surprised if he eventually decided he'd like to retire from the band in order to pursue other avenues. Sort of like "Okay guys, I've really enjoyed it, we've had a great run, but I think I'm ready to move on while I still can." Anyway, that's my take on things.