Well if that was the case, Woody Guthrie would have been a good candidate for it too! Certainly both Dylan and Guthrie took ideas and even lines and tunes from other sources. The argument on their behalf is that they enhanced those ideas and made something more of them. Woody Guthrie saw this as an inevitable part of the folk process.
The case for the significance of Dylan's work rests more on the way he tackled several genres of American song and was able to extend the range of what could be achieved in them. In particular, I can think of no artist before him, who attempted the complex psychological profiles which you get in songs like "Ballad of a Thin Man",or "It's Alright Ma, I'm Only Bleeding". You don't have to like him to recognise that he was artistically very ambitious.
Still, I had better shut up now. Little Hawk will be around at any moment - and he probably has the drop on most Dylanologists around here!