Two here. If James goes, I go. It would take a monumentally masterpiece of an album to keep me interested.
And to be brutally honest, while I still love the band and I don't have a single regret about seeing them on the last tour, and I was more than willing to go again even if Portnoy hadn't returned, the last two albums were the ones that had least staying power for me. They came and went and while I don't dislike them, I hardly ever returned to them, and even when they were new the times I played those album were dramatically inferior to the playing time the previous albums got.
So if in this equation, where I still like the band but the albums already don't have any lasting effect on me, you take away the defining voice of DT.... that's where I jump ship, unless they come up with the perfect replacement and a literal masterpiece. No, a quite good album with another singer isn't enough, it has to be a masterpiece to keep me interested.