This is all my opinion: JP and MP have always been the core sound of DT, what made them stand apart from other bands. I've felt vocals and keys were easily replaceable in the band. To me, LTE I and II are instrumental DT and are quasi-canon. Yes, JLB was the right guy at the time, and I enjoy his work all the way until the post-Portnoy era, but I think they could have gotten away with a replacement after FII. Honestly I totally agreed with MP that they needed a break after BC&SL, regardless if it was for his own reasons or if he truly felt like the band was burning out, as much as I loved and still love BC&SL, I also sensed the band was beginning to run out of fresh ideas musically.
I enjoy SC and BC&SL, but there's an overall 'sound' or 'essence' that went missing since Octavarium, and has only popped up in select, brief moments since.
Since MP left, the last 4 DT albums have more or less felt like JLB's solo albums with Mangini, but with JP and JR taking on the much of the writing.
While some may feel MP became stale and didn't keep improving his style and chops like JP, JM, and JR do, I think by keeping a consistent style, he kept the rest of the band grounded; whereas with Mangini, everything is very robotic, sometimes too technical with awkward flow and unnecessary-sounding time signature changes/added beats, and I've listened to plenty of prog rock. When he's just keeping a solid beat, it sounds uptight and not as relaxed like MP's style, and it doesn't help that I find a lot of the construction of newer DT songs to be of questionable quality, really just 'look at how many ridiculous things I can do in 30 seconds'... like, "OK, but is it something I'm going to want to listen to over and over for the rest of my life?"and I just don't care for the physical sound of Mangini's drum set, in studio and live, even on JLB's albums... Point is, I'm more excited about Terminal Velocity and LT3 than I was for the last few DT albums.