Speaking of singers and DT, and I know I'm gonna get flak for this, I consider James LaBrie de facto irrepleaceable.
Yes, there are scores of singers with a better overall live performance and stage presence than him. And there are many singers who could do justice to DT's songs, most likely some young guys in their prime could nail 10 times out of 10 the high stanza of Take the Time live.
But.... James has been on every single DT album except the debut. His voice is so ingrained in DT's sound and songs, that I can't imagine anyone else singing those songs and feeling "right". Yes, you'll find a long line of singers who will be able to sing the songs and have a better overall live performance. But after so many years and basically an entire carrer save a single, debut album.... no one else would make me go "ah, yes, this is still DT", even if Petrucci and Rudess are still there. A known singer, like, dunno, Russell Allen would feel like SX and DT together. An unknown singer would feel like "DT and some guy". A JLB clone would sound like a copy of the original. So I can't think of anyone other than James sounding like DT, even if they could sing the songs.
It would take a gigantic, monumental, undisputed masterpiece of a JLB-less album to keep me interested in a version of DT without him.