The notion of modern DT being "simpler and less inspired" feels a tad absurd to me. The arrangements of the last 10 years imo feel far more dense, deliberate and dynamic than most of the albums prior, to me. Is ADToE or DT12 simpler than Octavarium or Train of Thought? Despite an abundance of shredding in the latter, I don't really think so. Plus, it's not like every song was a blast of wild prog in the 90s either. You had ambitious stuff like Metropolis, The Killing Hand, A Change of Seasons etc. but also relatively "lazy" stuff like Pull Me Under, Afterlife, Caught in a Web and Burning My Soul. Not really any different to what we have today, with there being both stuff like Breaking All Illusions, Illumination Theory, At Wit's End and Pale Blue Dot alongside more straightforward songs like Paralyzed, The Enemy Inside (which I also think is more creative than it's been made out to be, I don't personally think "elementary" would describe it any more than it would for As I Am, The Mirror, Caught in a Web or Pull Me Under... which I also like) and Untethered Angel (which I'd say the same for).
It's not like TA is a simple song either. Three solo sections (counting the middle guitar + keys as one), a different final chorus, unusually angular melodies, an altered second verse, odd time signatures all over the place, MM going wild with all the syncopation and complementary rhythms, subtle usage of altered motifs... it's arguably as prog as it gets (and for me, in a tight and cohesive package that feels thematically appropriate to the lyrical content). Whether MP could do something similar is I guess beside the point, but I'd still go with a relatively confident no, despite my respect for him as a drummer. Even when MP did go technical, it seems (judging from what others have said, I'm no drummer) that he was locked in to a particular feel, while MM might have various layers of rhythmic activity. Not to say that MP hasn't done polyrhythms and stuff before, but on the level of something like Illumination Theory? I don't know.
As for "less inspired", that's one of those phrases that feels like it should be struck from discussions around art to me. I get "boring", because tastes will be tastes and what satisfies one person could leave the other thoroughly bored. The matter of assuming an artist is inspired is contextual guesswork. I feel like it's invalid because there are plenty of examples where a band has actually struggled with ideas and really forced their material out, but all for an album that ended up actually being pretty well received. The reverse has also frequently happened. Maybe it's just me, but I think the matter of inspiration should only matter to the person working on it. It's always felt like a rather brazen assumption to make that someone was uninspired imo, because it just feels like someone projecting their personal dislike of something as a character fault on the person who created it. Needless to say, I feel the same way about using "lazy" as a criticism.
The question of whether he can effectively improvise is valid and interesting. But from what (relatively) little I've seen, I don't think Mangini is lacking in that department. To give just one example, the band were VERY happy with his abilities during the improv/jam portion of the auditions.* And given that that is how they "write" a lot of their material, and that they were specifically looking for someone who could fit that mold, I don't think that's an issue.
This for sure. I think some of the later LTE improvs are pretty solid, but Take This for the Pain is probably the most interesting sounding piece of wholly improvised material I've ever heard. I also vastly prefer his interpretation of Finally Free's outro over MP's, which in comparison just sorta feels like a wall of percussive sound (not that there isn't value to that), rather than the interesting ways that MM plays with the beat for dramatic effect. It's maybe less that he "plays for the time signature" and that he's just naturally drawn towards angular, technical beats when coming up with stuff on the spot, which I think is perfectly fine and certainly befitting of the kind of band that DT are.