What Cash did with lyrics, Davis did with music. His musicianship was all he needed. Conveying emotions through music without lyrics can, and most of the time is, more powerful. I dig on Johnny Cash, don't get me wrong. But Miles Davis catalogue is fucking ridiculous. How he changed jazz music, and made several landmark albums that music critiques, not jazz critiques, claim to be as some of the greatest of alltime. Johnny Cash never truly had any great albums, aside from the novelty of San Quintin and Folson Prison, until he started working with Rubin. He just had certain radio songs that were catchy and ones that people could relate to. Aside from those radio songs, he didn't have that much else - Davis did have that.
Davis may have been a better performer, but people could relate much more easily with Cash, therefore they were more eager to get into similar music. His interview with Merle Haggard on the "Live from San Quentin" CD got a few people that I know listening to Merle Haggard, so all is well. And I'm not ragging on Davis, the man was a genius, but Cash was more accessible.