To me, changing singers is the ultimate gamble for any established band.
And that's really it. If DT did decide to change singers, they would be taking a risk - whether you think James is replaceable or not there is always some level of risk and singers especially are a huge gamble. When trying to decide whether the rewards are worth taking that risk, it really becomes obvious why they're probably not going to replace JLB unless he leaves on his own. 20 years ago when the band's future and creative direction was more of an open question, it would have made sense to replace JLB.
At best DT probably has one more decade left in them. They are way past the point where they are going to be bringing in waves of new fans and have settled into their sound to the point where a lineup change also isn't really going to significantly impact the musical direction. Even if all goes well, replacing James isn't really going to bring a lot of tangible benefits. Best case scenario is that fans who were already going to see the band live anyway are going to have nicer things to say online about the vocals. Worst case scenario is that the change rubs a significant portion of fans the wrong way or the new singer is unlistenable to a lot of people, causing their audience to shrink. I just don't see how it is worth the risk for them, nevermind the fact that they clearly all really like James as a person.
However, Stadler posted a big list of bands that struggled with changing singers. Without going through each band line by line, I think generally there is a lot of context missing:
No one is irreplaceable. Including James.
In the literal and phyical sense of the word, I agree. There are out there singers that can sing as good as James, guitarists who can play as good as Petrucci, keyboard players tht can play as good as Jordan, drummers who can play as good as Portnoy (case in point: he left and they got Mike Mangini) and bass players tha can play as good as Myung.
But as Trav86 says, after nearly 40 years there's no way any band would feel the same without a key member. Can Steve Harris be replaced in Iron Maiden? sure, there are tons of bassists out there who could play like him and replicate his tone and even his poses when playing live. That's what cover bands are for however.
Iron Maiden
Judas Priest
Skid Row
Motley Crue
Asia
Night Ranger
Boston
IMO the problem with these groups is that they changed singers at inopportune times and in moments where their popularity or the popularity of their type of music was starting to wane. There is also a chicken and the egg thing where vocalists likely jumped ship
because the band was getting less popular and they thought they could be better as solo artists. This was certainly the case with Iron Maiden to some extent IMO. I would argue that a lot of these 80s bands were never going to get through the 90s unscathed with or without their lead vocalists. It's not like Motley Crue did that much better with Vince Neil than John Corabi in the late 90s. What AC/DC and Van Halen have in common is they made lineup changes right when they were exploding in popularity. It worked for them because they were already unstoppable at that point.
Queensryche
IMO Queensryche is probably the closest example of what replacing James in Dream Theater at this stage would be like. Queensryche is well past the point of substantially expanding their audience and while they still make albums they primarily exist as a legacy act to play the "hits." I also disagree that they struggled with that change. Replacing Geoff Tate was the right decision and probably gave the band a lot more juice to keep going. I don't love the new albums, but they are undoubtedly better than what they were doing with Tate toward the end and the fans seem happy.
The Velvet Underground
Marillion (yes; they are not as big as they were in '85-'87 and never achieved that after).
Black Sabbath (except for Dio)
Deep Purple
Genesis
I think these are just verifiably incorrect.
This is all to say that I generally agree with the sentiment that nobody is irreplaceable. I think if they handled it gracefully and chose a new singer very carefully, they could replace JLB and be fine. The problem is that I don't understand why they would actually want to do that. It's not really going to impact ticket sales or change the way they write music. They're better off spending the next few years winding things down with their classic lineup.