it hasnt always gone horribly wrong,its just changed.,and there are some good revival singers performers ,who clearly still listen to source singers.
commercialism has played a part,plus the idea of listening to sounds rather than words,result: over elaborate accompaniments.,and on occasions thoughtless singing
but the real difference betwen Sarah and Tommy,is that she sung the songs all the time for her enjoyment while she was doing different jobs .
Tommy clearly didnt sing around the house,he practised his songs and performed them for audiences.,he clearly played for his own enjoyment as well,but he didnt sing while he was working at other jobs[otherwise Sarah would not have made the statement"I never heard Tommy sing]
so the attitude to the songs was different.
one other difference[in style] is that Tommy has a slight classical sound[possibly as a result of his choir training].
the other day I listened to the Transatlantic sessions[aly bain],all competent musicians,but the result [imo]was a bland mish mash of rootlessness,the music lacked something, the joi de vivre was missing