Every time I wonder about it, I feel like JLB can't be replaced just like that...Imagine from the other members perspective :
1) He has been there since I&W, his voice is the only known by most, like axl rose for gnr.
2) They are old and probably don't imagine changing their habits.
3) Maybe they think the chances someone not too young can do the work, at this age, without being a diva etc...
4) Low risk to wreck another part of the DT fanbase
5) Maybe they like the band writes album, then singer rehearses via webcam
6) True friendship over opinion
Cons :
-They hit a technical limitation like that
-Even if it's not like anselmo at the end of pantera, he seems not a lot physically with the band outside of tours.
-He puts W and Wo before voyels
All very valid points.
Let's remember our community here is a small world. For every user that posts that they can live without Pull Me Under, there are dozens of fans at concerts that will roar when the song starts. True, JLB has his fair share of critics but there would be an equally large (I daresay WAY LARGER) number of people that would jump ship without him. I am one of those, for sure - I would not "ragequit" the band if James were to go, but they'd better put a total and complete masterpiece of an album to keep me interested.
Also, which band with this longevity, and this stability with the lead singer, changed vocalist and remained at the top of their game? with due proportions, look at Judas Priest, they brought in a guy that could do (and scream) anything Halford could do and even more, and the fanbase still dwindled and people wanted Rob back anyway.
There's no scenario, at this point of their carreer, where DT changes vocalist and do even better than before. Best scenario? they play more or less the same venues they usually do with less people in attendance, and fans for a while enjoy the novelty of hearing Take the Time and Voices live like it's 1994 again. But replacing James would bring DT into "bands that still go on replacing members here and there while there's still people who want to see them" territory, the twilight of their carreer.
EDIT: Another aspect to consider, who would even replace James? prog metal is a small world, there is no singer out there who could potentially join DT and cause an earthquake of excitement and sales. Maybe Russell Allen, but still DT won't become bigger stars overnight. So they either get an unknown talent like Priest did with Ripper, a guy no one knows, or an estabilished singer from the genre. In neither of these two cases the prog world "breaks" and everyone jumps on DT's bandwagon.