1954 states that even a song with a known composer can be a folk song!

Says it all really

*Sigh*

Yes, and it also says how that can happen:

Firstly, the tune is to some extent translated into the accepted idiom, so that the continuity of tradition is maintained; secondly, it ceases to be static and stereotyped, but becomes multiform through the individual variations made by its performers; and thirdly, the forms in which the tune ultimately survives are determined by the community: for the variations which meet with approval persist, and the others die out. In this sense, a folk song, even when it has an individual origin, may be said to be of communal authorship.

The Manchester Rambler might be a folk song in 50 years' time; so might Eleanor Rigby; so might Oops I Did It Again. But they ain't there yet, and - barring the draconian intervention of Jim Knowledge's ministry of culture - they aren't likely to get there.