Stringsinger said ,Anonymous and PD are often the determinants of a folk song.
the problem with that is that no new songs get added to the tradition or become folksongs,unless the composer[does a BertLloyd]and sneaks his own compositions in as trad ,then of course when he gets found out the composer gets slagged off being intellectually dishonest.
the other problem with that statement is style,certain traditions have certain styles,irish music for example is predominantly in four modes,if one starts introducing tunes in the phrygian mode[ some flamenco tunes]they sound alien ,they sound more akin to a different culture/tradition, more akin to flamenco tunes,that doesnt mean traditions cant change,but often to be acceptable to the musicians performing them at that time theth introduction of a new mode has to be subtle and gradual,so that the process is evolutionary.