"1: Guitar distortion makes large chords sound "muddy", rendering something like a Maj79 functionally useless in most contexts, and thus uncommon in metal."
Distortion also adds a lot of overtones. A fifth 'chord' tends to have a considerable amount of harmony 'built in' by the distortion itself. Adding upper extensions serves only to replicate the function of other devices within the context of a metal song. The blast beat, extreme distortion, and 'crappy' production techniques tend to take over the function of harmony in metal. They set the background, context, and landscape over which the melody moves. Darkthrone's UFM and TH albums are two of the most explicit examples of this idea.
"Taking a basic C shape and moving it moving it to a D (as an example) on the guitar creates very interesting dissonances."
Bird Flu -
Darkthrone explored similar (albeit in a simpler way) ideas on TH (probably on others, too, but I am only really familiar with the 'written' music of TH). Metallica also does this most notably on the intro to the song "Master of Puppets." Both bands sacrificed on the harmonic complexity that your example would bring for greater focus on the riff and for the reasons I've stated above about the harmonies that already exist within a distorted note, fifth chord, or chord. Ideas more akin to your example are widely used in pop music. Smashing Pumpkins "I am One" and Bob Dylan's "Tangled Up in Blue" are good examples. While they may have more harmonic depth than a metal tune, the harmonies ultimately act as a spotlight for the vocals/lyrics.