They've got to at least make it their 50th Anniversary tour. That's one where I think Portnoy would be performing on and one to celebrate big with. Maybe make it a big event with two, or even three, days dedicated to being a special show, something that could be done towards the end of the tour leg/cycle.
JLB is integral to what JP, and likely the rest of the band, consider the Dream Theater "sound." I can see their side as well, since they are also the ones playing beside him. It'd probably be pretty awkward after all these years, playing with a different singer/front-man.