I meant to say that you could in theory write a contemporary song that was readily adaptable to suit difference circumstances, contained a simple chorus, circular motifs, etc., which would make it much more likely to become a folk song within a generation or two.
What makes current songs unlikely to be taken up as folk (by purely traditional definitions) is that they are generally recorded and offer a perma-text for comparison which militates against transformation and the desire for recognition by the writer/singer.

It's not beyond the bounds of possibility for relatively new material to be passed off as folk, given a reasonable enough pastiche and some hidden provenance of the 'collected from the diaries of an old man in the village' variety.