So I say all of this as a drummer of the last almost 3 decades, who has been into DT since before 6 Degrees was released. I'm also a drummer who didn't learn by studying rudiments and such, but by learning songs and just trying to be able to play what came into my head when writing. So that probably biases me a lot.
I hear the complaint (or observation) a lot that Portnoy resorts to the same bag of tricks and they prefer MM who practiced a ton and was more technical. I actually agree that Portnoy eventually came to have a bag of tricks, mostly in his fills. He has maybe 5 fills that he reuses a whole lot with variations, and some times he does fills that aren't part of that. But he does the same fills so often that we notice quickly. His beats are also very much in his personality. So they mostly stay true to his style, rather than constantly looking for new ground to explore, though I hold he does that too to a degree.
However, MM doesn't sound much different to me. Yes, he brought in a different drum philosophy to DT (one which did not resonate with me) and maybe all of his fills are radically different if you break them down and study them, but to me a whole lot of them just sound like a fast flurry of hits. So it doesn't matter to me how technical it is or exactly what he's doing, if it all just sounds like a flurry of notes to me in the end. He also hasn't really veered much of his bag of tricks. Play polyrythms, double the guitar/bass with one set of limbs and double the keyboard with the other. It got boring for me really quickly because it felt like a novelty that didn't stop. It just didn't connect with me at all. He did some really cool things and did bring some neat innovations, but I honestly listened to the last 4-5 DT albums a number of times and rarely did the drums stand out as new and innovative without having to study what he's doing and break it down.
Also, I don't see much innovation from anyone else either. Yes, conceptually the Astonishing was different, but riff wise and melody wise, not a lot terribly new. And on the other MM albums, a lot of their stuff is also from a similar bag of tricks. Petrucci solos have become quite boring to me in the last few albums because I always know what he's going to do because he's just doing the Petrucci thing. JR similarly, and let's not pretend JM is innovating on bass.
So to solely put this criticism on Portnoy when it seems to mostly apply to everyone in the band, though it did likely apply to MP first, seems a bit disingenuous. If you don't connect with his playing, cool. But let's not pretend the rest of the band are constantly growing and learning and MP isn't.