Handel's Messiah not folk music? Not too sure about that. Listen to the Carol singing tradition in the villages round Sheffield, it's essentially part of the same thing as Handel. It would be perfectly possible to sing the Alleluia Chorus in a good folk crowd. It could sound great. Look how Carolan survives the most remarkable performances, and still comes through.
Here's something I posted in another thread ("Original Music that sounds Traditional") that seems to fit in here (if we can't quote ourselves, who can we quote):
How come we keep on talking about "the tradition". There are all kinds of different traditions, some of them longstanding, some very recent indeed. For convenience we bundle a few of them together and call that "folk", and then argue as to which ones should be included, and what are the common factors linking them.
While a tradition is alive, new stuff can be produced which is part of that tradition. Once it's dead, the songs and the music are still available, but new stuff in the same style aren't part of that tradition, they are part of another distinct modern tradition.
So Sea Shanties came from a tradition which involved them having a role as work songs. You can't have new songs which are part of that tradition, because that's not how working boats are worked any more. If you make a "new sea shanty" it is something else, even if in form it looks and sounds like a sea shanty and it might be a great song.
Just as you can have reproduction furniture, and it can be good furniture, you can have reproduction songs, and they can be good songs. But I don't think it's right to artificially age songs to pretend they are something they are not. The exception I suppose is where for a particular purpose, such as a play, a song is written "in costume" - that is where John Tams and people like Graham Moore come in. And it is easy for songs like that to be taken as taken as traditional. I'd sooner use the term "in the tradition" (meaning in some particular tradition).
But generally the best songs are songs which may draw on traditional elements, but don't dress up. Stan Rogers, for example. I'd say these are songs coming out of the particular tradition, rather than as being in the tradition or traditional.