What I mean about the Messiah is that, just as it is possible to play Carolan either as classical music or as session music, so you can do the same with significant elements of music by other classical composers - and I think we should feel free to do that. I've been in sessions where tunes from Mozart of Beethoven have cropped up, and felt quite at home.
In other words, different traditions can draw from each other. It's always been recognised that the Classical traditions make use of tunes from folk traditions. But I don't think it's always recognised how often the reverse process occurs.
It seems evident to me that the best way to think about these matters is to recognise that there are an enormous range of different musical traditions. And I use the term tradition in a very broad sense. Eveything fits into some kind of tradition.
We can gather together conceptually a lot of them and call them "folk", but some of the ones we will include will be very different, and may have more in common in some ways with others that we don't include within the folk family. And there will never be any agreement about which traditions should be included within the label, and which should not. That's why this discussion is never going to come to any firm conclusion. We can either decide on the criteria for calling something folk and that will determine what we include within the umbrella term. Or we can decide what we want to include, and then work out what they have in common.
But what I think is more interesting is to try to work out the relationships between various traditions - how hymn singing shaped the way songs were constructed and sung in some times and places, how dance music shaped song patterns elsewhere, how different traditions collided and effected each other as people's circumstances changed with emigration and immigration and so forth.