I would remark that Johnny Cash's vocal range was something less than a whole octave. Yes, he wrote songs--some good--but I once walked into a bookstore playing an album on which he covered other people's songs. That was odd.
I guess today we regard some canonical rendition of a song to be the 'correct' one. Does that stem from the dying art of singing - folks don't hear songs except in their most viral form?
By all means, one should sing old songs, and for many purposes remember Chesterton's dictum that 'anything worth doing is worth doing badly'. But if one is going to market an album of X sings Y, it is well if X actually can sing Y close to tune.