there are a lot of very intelligent rappers with a lot to say in the way of social commentary and philosophical undertones in their lyrics. for instance, the lyrics of RZA, GZA, Aesop Rock, to name a few (worth noting that these three rank high on lists based on the most extensive vocabularies amongst rappers). Lauryn Hill and the Fugees jump to mind as well. I can't help but feel that your blanket dismissal of non-classical forms of music is based on a limited, narrow understanding of music overall (also let's not pretend all classical music began as highbrow--much was written by commission as "party" or chamber music for royal courts, or to commemorate Emperors and such). Rap is essentially a new form of folk music. Folk music throughout the last hundred+ years has often served a valuable purpose of questioning culture and social norms. It is hardly just there to debase and devalue prior tradition, although from some perspectives, it might do that at times. However, I would argue questioning and subverting the old is a healthy process in art and literature, lest we get stuck treading water and canonizing "the classics" at the cost of appreciating new ways and means of expression.
Spoken cultures have rich and complex languages, and complex social structures, even while they are illiterate, and are the birthplace of the tribe.
While literate societies based on the phonetic alphabet and musical notation, are the birthplace of the individual.
Universal literacy in prosperous societies is the birthplace of the Western Enlightenment of evidence and reason, of evidence based medicine. evidence based science, free speech and liberal democracy, evidence based economics, evidence based law, and the Universal Declaration of Human Rights.
Spoken cultures resent being merely the content of literacy, as in Anthropology, and hate literacy, and demonise literacy and blame literacy for the faults of spoken cultures.
Spoken cultures think intuitively, while literate cultures think counter intuitively. So the two cultures think differently.