"If Ewan's songs are not folk songs then, does that mean that performing them at folk clubs is assisting the demise of folk music?"
You really have not been listening have you Dave ?
MacColl and those around him were not antiquarians who claimed that only folk songs should be performed on the scene - on the contrary, the argument was always that if this happened the clubs would be no mor than museums
MacColl used the tradition to create new songs - more than any other singer on the scene
Peggy produced 20 books of songs (around 20 songs per book) made up of contributions from singers and songwriters from all over the English-speaking world, as far afield as the US and Australia   
As far as I know, none of the composers claimed that they were writing real folk songs
The Singer, The Grey Cock in Birmingham and other clubs that followed suit created venues where traditional and newly written songs worked together in harmony because they were related in form
Ther was a strong encouragement to research and to set up workshops for new singers
A far cry from the present use of the term to cover anything from fifth rate pop to whatever people wished to call folk - a progressive experimental music based on traditional forms
While MacColl insisted he didn't write folk songs, he made sure that what he did write and sing didn't clash with the traditiona songs and ballads that dominated his, Peggy's and the rest of the residents' repertoires - Jack Warshaw, Sandra Kerr, Dick Sneel and Phil Colclough - all residents, wrote some excellent songs - a far cry from the cultural dustbin today's scene has been turned into
Even Bert Lloyd turned his hand to song-making on occasion
It was no accident that that none of Ewan's clubs were ever called 'folk clubs' though that's where you went to hear good folksongs well performed - they had a world-wide reputation for being just that
Jim