Well I will agree that rhythms have gotten pretty crazy in recent history, which is part of the reason I enjoy listening to King Crimson and Symphony X. But come on, classical music is way more advanced as far as counterpoint, thematic development, timbre/orchestration, and very often in technical skill as well. And they didn't "need" a lot of instruments - there are countless classical pieces played as sonatas, solos, and concertos; you cannot limit classical music to late-18th century symphonies! There is so much more to it than that, and the tradition of western art music continues to this day in styles that are far more complex than anything rock has ever done! You want to see technically complex, look into integral serialism - composing those pieces are mental workouts, to say the least, and they incorporate almost inhumanly difficult timings and changes in dynamics! Besides, even when a lot of instruments were used, it was generally for good reason, not arbitrary or superfluous.
I like prog metal, but I just think its modern incarnations like Mastodon, Cynic, Opeth (sorry to the fans of them here), Dream Theater, Pain of Salvation and Between the Buried and Me suck. They're disjointed, and every phrase they play is replaceable with every other one. There's no direction, feeling, or accomplishment. (And this is coming form someone who loves technical music as much as anybody) The only bands that I believe have adopted well to the modern prog metal sound are Fates Warning, Sieges Even (R.I.P.) and Symphony X.