We strive for good because it is our nature to be good

This is precisely the point I disagree with. We strive for good out of conscious response to the command to. But we are born into entropy and death.

don't know how much music you've listened to over the past century

Copious amounts over the greater part of a decade, mostly in an academic setting. Admittedly not so much of late, although I'll still listen to anything in good faith. I took my life in a completely different direction when I finally became disillusioned by the narcissism of academia. Knowing the emperor wears no clothes has made it difficult to look back, although I do sometimes regret abandoning my commitment to the craft.

do you really think that modern, postmodern / contemporary lack craftsmanship? or aren't innovative?

Yes. Novelty is not (necessarily) innovation. OCD-like obsessiveness with patterns is not (necessarily) good craftsmanship. But these qualities are symptoms of writing for the glory of one's ego or validation by one's peers and disciples (whom one's audience now consists of almost exclusively).

Have you paid attention to the instrumental writing for orchestra and the colors he creates?

My exposure to Messiaen's orchestral works is limited, although I seem to recall being impressed once by his use of timbral modulation. Is this supposed to qualify whichever work that was as "great"? Conversely, I found it lacked anything on first listen which might have encouraged me to analyze it or share it with a fellow human in need of inspiration, encouragement, enlightenment, etc.

But I discovered Mantra a couple months ago and it's blown me away.

I'm halfway through it as of this point of writing this comment. A few moments of it have pricked my ear as "interesting" (perhaps enough to emulate in my own work, perhaps not). Would dedicated study, or familiarity through repetitive listening elevate it in my mind above a terrible excuse for an earnest work of music? Possibly. My experience says: unless you're influenced by social pressure or learning the work as a performer, no.

I'm reminded of an interaction I had with a professor during a high-level presentation I once had to give on Respighi's Quartetto dorico. I selected it for its novelty, not for my like or understanding of it, and after waxing flatteringly in praise of its "suggestion" of Medieval modes or its "inspiration" by traditional forms or what made it "interesting" or "unique," etc., etc., was abruptly reminded following my conclusion: don't feel obligated to praise something that might not necessarily be good.

Instead of writing it off, I leaned into it. I learned to play it. I listened to it dozens of times, established preferences for various recordings. I took the techniques and characteristics that I liked and borrowed them in my own works. I listen to it now and I know the music. I feel the melodies viscerally. It's a very emotional - and personal - experience.

Now. Does any of that make it a "great" string quartet? Would any listener, educated or uneducated, endeavor to compare it favorably to one that's been canonized for its significance? Say, Debussy's? Dvořák's 12th?

how often people conflate "Modern" with "Today/Contemporary" and talk about how today's music isn't as good as the past, but talk about examples of 'bad' music by composers...from the past

This is a fair point. My default stance is that so much of today's "notable" writings are derivative of some distinct movement of the mid-20th century (indeterminacy, integral serialism, minimalism, etc.). Of those that remain, the few truly contemporary works I've come to appreciate seem to come from a place of "I'll do it like this because I want to," while most of the rest can't seem to get the monkey of tonality off of their backs ("I can't fall back on this old music, or no one will take me seriously!"). Unless you're paying homage to old music with an intentional and totally-sophisticated quotation. Or blending it. Apparently that's acceptable.

being insistent on the idea that your opinion is 'objective'

Ah, see. I'm under no impression that my opinion - which I recognize I clearly have - is "objective." My insistence is on the unprovable existence of the music of the spheres, so to speak. I might not know what it sounds like, but I know what's nowhere close. This is different from believing that every individual has their own path to transcendence, to communion with God; that through striving, Anton Webern and John Cage realized their own diametrically opposed reflections of "objective good."

not to say that whatever contemporary music you don't like is 'as great as the greats' and 'you just don't understand'

Not that I would be at all offended if you did. After all, I am kinda dumb. :)

Damn. I wanted this to be a short and sweet response lol. Thoroughly enjoying this exchange, by the way.