Isn't it simply the case that as generations age and pass on, the people who learned those songs from "source singers"become the source singers for the next generation? Isn't that the oral tradition. It has only been the advent of collecting and then modern recording that has enabled people to be labelled as the "source singer" for a particular song. Surely with time the distinction becomes artificial?
For me, the most important thing is a sense of authenticity and a connection made between the singer and the song which is, in turn passed on to the listener. The point made about Jean Ritchie is absolutely right and can be made about a whole number of other singers.