Responding to a couple of Pip's comments:

Yes I agree that the folk process is going on only among a fairly small community -- but I strongly suspect that the people who remember songs (especially songs from past centuries) sufficiently to sing them in more or less their entirety, and who change them consciously or unconsciously, have ALWAYS been a small proportion of the population.

When the collectors went out around a hundred years ago, they found very few singers, and thought those were the last survivors of a much greater number in the past, but I know of no evidence that the number actually had ever been much greater than it was then, or is now.

Richard