Difficult to say. I think that his great contribution to country music was as a bridge between the Woody Guthrie influenced country/folk music of the likes of Kris Kristoffersen and Bob Dylan, and the Nashville scene.
He managed to be intensely political without being a soap-box leftist, or even admitting to any politics at all. The pure humanity of his recordings, and his belief in redemption stand him apart from every other artist I can think of (particularly Man in Black, Peace in the Valley, Ira Hayes and Folsom Prison).
I'd say that without him, country could have descended into a kind of Nashville/Memphis Pop (Garth Brooks style vomit), with American Folk becoming a separate genre (I know it is now, but you would find bands who today identify as country or alt-country calling themselves Folk).