First of all Kurt Cobain was not a great artist. He was a popular artist, as was Andy Gibb, but great? Come on, that would be putting him in the same category as Bach or Mozart. Now they were great artists. As for Cash, he qualified his judgement by stating that he was not an origional artist. Most of Johnny Cash's hits were written by other people. Yes, his interpretations of those songs were well done, but this does not make him a great artist. I will give Johnny Cash more credit than the author did, but I won't disagree with his opinion. Yes, he pointed out that Johnny Cash was a drug addict, and that may have caused him to be unable to write his own music or lyrics, but again that is not necessarily an untrue statement.
I like Nirvana, I like Johnny Cash, but they most certainly are not in the relm of aritists such as Buddy Holly, Elvis Presley, or Bob Dylan. Those people wrote and sang their own songs and didn't need anyone else to underwrite their success.
Elvis Preley DID NOT write any of his hits. I don't think he even wrote his own album cuts.
I guess Sinatra doesn't make the grade, either.
I would bet that Cash wrote more of his songs than Presley did.