Josh,
If I am understanding you correctly, you are saying that art can either more or less conform itself to what is true, good, and beautiful. Some things that call themselves art do not conform to the true, good or beautiful very much at all. Other works of art conform much more so. I hope I am understanding your basic point correctly.
I agree with that. Of course, I would say there is a spectrum of conformity. It is not a matter of simply whether it does or does not. Since works of art are created by us limited human individuals, no work of art – no matter how sublime – will conform perfectly to the True, Good, and Beautiful, which is God Himself.
I do not think that it is fair or honest to paint the entirety of rock music with one brush and say that it is impossible – by nature of the genre itself – that any piece of rock music could ever conform to the true, good, and beautiful in any capacity whatsoever. In my experience, that is not the case.
When I listen to something like
A Change of Seasons by the band Dream Theater, I find lots of truth, goodness, and beauty. Indeed, I find it somewhat transcendent.
Now, as to the claims about the “satanic origins” of rock, I find that argument lacking on several levels. First, I am very skeptical about the internet conspiracy theories. They seem to paint the picture of one satan-worshipping guy to whom Satan was whispering in his ear coming up with “rock music” by crafting a genre deliberately to bypass the will of all who hear it so that, without their knowledge or consent, they will – with utmost certainty – be led to hell merely by listening to it. Sorry, I don’t buy it. It sounds more like the plot of a Jack Chick tract than a factual account of the history of rock.
But I haven’t really looked into the origins of rock myself because, frankly, it doesn’t matter. Church history is full of examples of the Church taking pagan things, holding up what is good in them, and using them for Christian purposes. Even if the originator of rock music had evil intentions, that doesn’t make the whole genre perpetually evil.
As to the concept of “simple chords”, I would again point you to the above Dream Theater song. There’s nothing simple about it.

But regardless, there’s no reason that art cannot still be true, good, and beautiful even if it is simple. Indeed, there is much truth, goodness, and beauty in simplicity.
Finally, I would say that trying to say something uniform about all “rock music” is a bit like trying to nail jello to the wall. There are so many genres and sub-genres of all types that fall under the “rock” umbrella. I don’t see how one could condemn the genre of rock music unless one is willing to condemn the use of rhythm in music. Because that’s about the only thing that it all has in common (and even that is iffy when you consider progressive rock).
That’s my take on it. I look forward to any insight you have to offer.