Jim Carrol, the history of popular music is littered with examples of songs manufactured to be a 'hit' so I'm afraid that argument does not hold water! And of course you can write a 'folk' song or tune. If a piece of music is written paying respect to the tradition of the country of it's origin, then that is the folk music of that country as it maintains and develops that tradition.

The point above about instruments is also valid, few of the instruments used to accompany folk are native to the British Isles, so if we are not precious about the instruments, how can we be precious about the songs? The late Tim Hart made the point defending critics of Steeleye a few years back, that at the time the songs were first written they would have been sung either unaccompanied or accompanied by whatever happened to be available at the time, and in using electric instruments, steeleye were merely doing the same.

So the only difference is that we have traditional and modern folk songs and tunes, someone wrote the traditional songs in the first place, and the most important thing is to maintain the tradition, and new songs and tunes is how it evolves!