Instinctively I'm drawn to period music because it reveals things, often between the lines, about the people who made it. Folk music is worth preserving for those reasons, though it's not uniformly 'good' and I stand behind the judgement of modern sensibilities in being able to tell the difference. I can see why an Alfred Wallis maritime painting, an authentically naive but informed work attracts high price tags and can appreciate folk music that comes from a similar well spring, unsullied by the influence of more diffused minds.

Like say, architecture, it has to be recognised that music is not a pure phenomenon but a palimpsest of tastes. Once those tastes are added to the original intention is occluded, we accept them as an artifact that arrived with us at a moment in time like we might take a strata of subsoil that contains C19th stuff knowing deeper earth may contain medieval, anglo-saxon, roman or prehistoric pieces.

It's important to recognise that what we seek to preserve is also an act of taste, of connouiseurship and like despised Victorian buildings in the sixties, taste is fleeting. So is the answer to preserve everything? Probably, but in the knowledge that doing so will only tell us about taxonomy. I don't believe Tam Lin was under threat because Sandy Denny performed it through a mic with an electric band, if anything it spread the song to another audience who may have been moved to discover more about the song or folklore generally. There are enough collectors about to set in stone, or whatever virtual rock Wikipedia consists of, traditional songs and the songs themselves are robust enough to weather whatever interpretations a contemporary player may lend them. Imo, we're more likely to kill folk with kindness, to put newcomers off by presenting it purely as a historical re-enactment, than we are to let the song's robustness be tempered by new attitudes.

On Santa's last point, you may exhibit peculiarly liberal attitudes to folk music but rest assured many do see it as a hotline to some musical mother lode and believe it was a common currency rather than a fascinating branch line of popular culture. On Sedayne's point it's unfortunate that folk trades on its anachronistic qualities, much as I might appreciate them, because like patriotism it can be a quirk that negates further discussion, and there are a fair few on Mudcat who'd like to keep it between the boys (though not you Sedayne from what I've read) or want to restrict it, and debate of it, to accolytes and an agreeable elite. In the end folk's unique selling point will preserve it despite the woolly jumpers, dreary monologues and hands-off proprietorial nonsense.