"no one will go back to the authentic but inferior version, but still, the song retains its original form for centuries"
Sorry to disagree John - even assuming that anybody knew what the "authentic" version was, inferior or otherwise, that wasn't how it worked.
The survival and dissemination of these songs was entirely down to the universality of themes that enabled people from all over the English Speaking world to adapt them to suit their own situations and surroundings - a song from a fishing community in East Anglia was often adapted to suit say a farming area in Somerset, for interest.
That is, or rather was the 'folk' process, and it is basically what we have lost.
Forgetting and mishearing certainly was a part of the re-making of these songs, but the remade versions became just as 'authentic' as the original ones (perhaps "earlier" is a better word")   
I believe that "Improvement" was much more the trade-tools of the early collectors and anthologists rather than 'the folk' themselves.
Jim Carroll