I love LaBrie, I realize that he's the voice of DT, and if he left DT I'd feel like something was missing and the band really wouldn't be the same.
That said, James has had bad luck with his voice leading to live inconsistency and, while he's improved a lot (which is a testament to his dedication to healing), his range and the duration for which he can hold a note still have been affected.
Is it so bad to imagine what DT would sound like with a singer who's voice matches the consistent high level playing of Rudess, Mangini, and Petrucci?
I wouldn't want DT to replace James, but if they did, I wouldn't want them to go with another "rock" or "metal" vocalist with a voice that probably suffers from the same wea-and-tear issues as JLB. I'd want them to select a powerhouse vocalist who was just as technically gifted and consistent as Rudess, Mangini, and Petrucci.